I don’t consider myself a “humanist” (even in a marxist context, I just don’t consider it to be a useful way to call that philosophy) but not because humanism supposedly dehumanizes humans that are not human. I think humans aren’t necessarily exclusive when it comes to being creative.
But I’m not arguing for the other perspective. It’s not about that “humanist” theory being based upon centralizing around humans as being a superior and utmost advanced being. It’s about addressing our detachment from how we interact and our relations with the natural world.
To be isolated from our species being is to be separated from our class, our material conditions, and what us as creative beings serve and function. To be anti-human as a theory seems like it comes from a deeply resentful and anti-social place.
But I won’t judge anyone for being holding an anti-human sentiment now. We are after all the bearer of the destruction of all species beings, including ourselves. Nature is suffering from our detached subjects.