Atheism, scientism, physical materialism are close enough to each other philosophically that the distinctions aren't that substantial.
The key thing is that the vast majority of bona fide atheists believe life, the universe, and everything can be explained by the motions of subatomic particles - by materialism.
Whenever they start veering off into some kind of quasi-pantheism then they've left the confines of the atheist reservation, and are tacitly confessing that physical materialism probably isn't a sufficient explanation for life, the universe, and everything.