Nicely said. I think you get to the heart of it in your third-last paragraph: being an atheist is different from being an atheistic activist. Activism is bound to be tribal, and atheistic activism is bound to politicize atheism and religion.

So your disillusionment is with the political aspect of being involved in an activist social movement. That has little to do with atheism, although it might be especially disappointing to realize that atheists or freethinkers can succumb to tribalism too.