Atheism does only mean lack of belief in gods. Humanism means a philosophical stance that focuses on the value and agency of human beings. However atheism and humanism accord with one another very well, which is the reason that a lot of people are atheists and (secular) humanists at the same time.
Humanists believe that human experience and rational thinking provide the only source of both knowledge and a moral code to live by. ... Humanism is a democratic and ethical life stance, which affirms that human beings have the right and responsibility to give meaning and shape to their own lives.
Humanism is a philosophical and ethical stance that emphasizes the value and agency of human beings, individually and collectively, and generally prefers critical thinking and evidence (rationalism and empiricism) over acceptance of dogma or superstition.
Rather than label myself with some grand “ism” I’d rather just say that I highly value humanity, or that I am intrigued with Ultimate Reality but think traditional gods are just mythical.
In the final analysis all these “isms” represent nothing but personal opinions, of little significance.
You are correct on a basic level, all of culture is intersubjective belief so humanism is just another ism. But at least it is based on something that is reality, that is not a superstition.
I am atheist today because I found that in my earlier life I was duped into belief in a god. There is no evidence for any gods at all and therefore we just might have made them all up. Humanism and the very word "humanist" seem to come out of Evangelical religion to me. I cannot tell you what humanists believe. People are all different. What is the only source of knowledge and a moral code to live by? IDK. Is there only one source? What I do know for sure is that theists believe they have a "moral compass" that comes from their biblical and tribal warring god. That would never work for me.