The qualifier isn’t terribly necessary most of the time. Most people don’t draw a distinction between ag/nosticism because it’s pretty personal generally. I consider myself agnostic given that I cannot know for sure, but ultimately, people that logic themselves into atheism (and thus are gnostic atheists) are no less valid.
That said, antitheism is distinct from the other terms and I contest the generalization that people assume atheists are antitheists, but that’s gonna vary a lot by how religious a region is (the more hardcore a person is, the more they see atheism as being an attack on them). It’s a bit of a bad subsetting issue. All antitheists (excluding the rare individual that believes in one or more gods and believes they should be broght down like an anime or a progression fantasy novel) are atheists, but not all atheists are antitheists.