I don't recommend the use of any plastic in your soil,ash, lime, straw or peatmoss work far better and help generate a healthy soil.

If your garden gets flooded the styrofoam will float to the top .

What you saw was definitely vermiculate, as already said. But I have also used stryofoam, but never for food crops. Not sure of any toxicity threat, but just in case. I used it routinely in pkace of gravel at the bottom of potted rose pots. At 20 to 24 inch diameters, those monsters were already heavy enough without adding a layer of rock.
Styrofoam is definitely a little bit of a compromise, but it was one way for me to repurpose much of the packing material that tends to accumulate from all household items that come in cartons.

The white bits are perlite. It a form of volcanic glass. It’s use in potting mix because it is porous and captures nutrients and water the plants will need later. It also aids drainage. The only this styrofoam may do is help with drainage.

Rough coarse bark is more natural and will do much the same job, grit is good too and adds weight so it can stop the containers or the compost blowing about in the wind.

Nope... not styrofoam. I believe it's vermiculite.
Helps control water evaporation.

Seems like a great idea. I hope it works out for you!