/lit/ - Literature

I am confused by what my philosophy teacher said about being atheist or agnostic

I learned some time ago from this video: https://youtu.be/qeFjLcQNqV0?feature=shared that one can be an atheist agnostic who doesn't believe in god and isn't sure about it's non-existence.

So in class (virtual classes, so I couldn't argue much, because there wasn't much time to do so) we were learning about gnoseology, and I asked the teacher about the difference of an atheist gnostic and an atheist agnostic after he said that atheists deny the existence of god and agnostics just don't confirm or deny it.
He said that one cannot be atheist and agnostic at the same time, because those are opposing views.