Evidently not a new discovery, although probably most of what appeared about it has been in Polish. An article about megalith tourist destinations states:
In a forested area about 1km from the village of Wietrzychowice in central Poland in the Kuyavian-Pomeranian voivodship, are megalithic buildings commonly called the Polish or Kuyavian pyramids, which are 5500 years old (3500 B.C). This is a cemetery from the funnel-beaker culture period, where human skeletons were found. It consists of five megalithic monuments – kuyavian tombs. They have a trapezoid-triangular, markedly elongated form, reaching 116 m. The fronts of the tombs are directed to the south and reach a height of about 2 m. The whole structure is stabilized from the outside by stone boulders. In the middle of the front wall there is a gap in the boulders, which is the entrance to the burial chamber. Traces of cannibalistic feasts were discovered there [1, 6, 56, 57, 58, 59]. Due to its uniqueness, the area is potentially a crucial cultural element for the region, which can contribute to the development of archaeological tourism in the region [56, 60]. In 1968, an Archaeological Reserve was established in Wietrzychowice, and in 2006 the Wietrzychowice Cultural Park, with an area of 37.65 ha was set up. In the Kuyavian-Pomeranian voivodship program of Heritage and Cultural Landscape Protection for the years 2000-2012, the Wietrzychowice Cultural Park was added to the list of Monuments of History [61].
[degruyter.com]

I would question the veracity and qualifications for the article. The source is wuestionable.

The Daily Express is a daily national middle-market tabloid newspaper in the United Kingdom. It is the flagship of Express Newspapers, a subsidiary of Northern & Shell (which is owned by publisher Reach plc).

It was first published as a broadsheet in 1900 by Sir Arthur Pearson. Its sister paper, the Sunday Express, was launched in 1918. In February 2019, it had an average daily circulation of 315,142.

The paper was acquired by Richard Desmond in 2000. Hugh Whittow was the editor from February 2011 until he retired in March 2018. Gary Jones took over as editor-in-chief in March 2018. The paper's editorial stances have often been seen as aligned to the UK Independence Party (UKIP), Euroscepticism and other right-wing factions including the right wing of the Conservative Party.

On 9 February 2018, Trinity Mirror said it would acquire the Daily Express' parent company, Northern and Shell Media, in a deal worth £126.7 million.

In addition to its sister paper, Express Newspapers publishes the red top newspapers the Daily Star and Daily Star Sunday.

[en.m.wikipedia.org]

Let's have a look a little closer at this article...

"ANCIENT tombs predating the Egyptian pyramids in Giza have revealed a time of prehistoric horror and cannibalism in the Stone Age, archaeologists have claimed" - which archaeologists? Which university or commercial archaeological unit are they associated with? It'd be nice if we were given the chance to check out their academic credentials.

"Pyramid-like structures hidden across north-central Poland" - they look far more like a local style of long barrow than pyramids to me, largely because they're the same shape and of similar construction to long barrows rather than looking anything like pyramids. Long barrows are very common in Northern Europe, with some 40,000 known to still exist today. Compare the one depicted in the article with Wayland's Smithy, a well-known example in Oxfordshire: [en.wikipedia.org]

"bloodcurdling neolithic rituals" - their culture was very different to ours, and to the people who built these tombs consuming parts of their dead may have been very logical: perhaps, as some tribes still do, they believed that by eating chiefs and shamans they would absorb some of their strength and wisdom, and if you're living at subsistence level with your tribe's survival depending on whether or not the next hunt was successful, eating the most nutritious bits of dead people makes some sense - certainly more than wasting it by burying it in the ground. Yes, the concept seems a bit strange to (most of) us in modern times, but this choice of words shows poor understanding. Further on, it seems that Sebastain Kettley, the author of the article, has derived most of his details from information boards displayed at the site, which can be somewhat sensationalist, rather than via proper research. This suggests that he has little background in science or archaeology, and a little digging confirms that while he does have a degree in journalism this is indeed the case.

"the tombs were built between the 4th and 3rd millennium BC" - so which is it? With all those bones and (even more reliable, since ancient burial sites were often used over long periods of time) organic material buried and preserved within the structures, it ought to be a relatively simple matter to obtain a more accurate date. This also brings "believed to predate the Great Pyramids of Giza by thousands of years" into question as the Giza Pyramids are themselves 3rd millennium BCE, with the Pyramid of Khufu (also known as the Pyramid of Cheops, the name by which the Ancient Greeks knew Khufu) dating to around 2570-2550BCE. So the Polish structures may be older than the Great Pyramids, but might also not be.

"This was the time of the Stone Age, or the neolithic" - actually, the Neolithic is the New Stone Age, which is only one era of the Stone Age. It was predated by the Mesolithic or Middle Stone Age and, earlier still, the Paleolithic or Old Stone Age.

Finally, let's take a look at some other stories recently published by the Daily Express...

Antarctica shock: Mysterious blacked-out structure vanishes on Google Earth - sparks alarm - [express.co.uk]

UFO sighting: NASA Mars rover spots ‘ancient black alien orb’ floating over Red Planet - [express.co.uk]

China's Loch Ness monster: Mysterious creature filmed emerging from world's biggest dam - [express.co.uk]

Asteroid warning: Deep impact predicted for 2069 claims Nostradamus prophecy expert - [express.co.uk]

It should also be noted that the Daily Express has frequently been criticised for publishing a large number of advertorials, articles which are presented as editorial content and thus circumvent rules regarding claims made in advertising, but are in fact "sponsored" by the companies of products given favourable press in those articles. In other words, it could be argued that it's not really a newspaper at all, merely a collection of adverts interspersed with lots of clickbait. This article is, in my opinion as a qualified archaeologist, not even worth using to wipe your arse.

Click bait.

There is no strong evidence, in this article, that unequivocally confirms the hypothesis. It smacks of an attention-grabbing headline, which undermines the article as a whole.

Yikes