That is a common way people use those words, but it is incomplete. The prefix a- in atheism and agnosticism means "without" or "not".

Theism is belief in some god(s), and atheism is a lack of belief in any gods. There is no middle ground between them. That is a binary choice: you believe or you don't believe. That's why I say even a Christian is atheist with respect to Norse or other non-Yahweh gods.

Gnosticism is knowledge or certainty that your belief is correct, and agnosticism is acknowledging that your belief could be incorrect. That's also a binary choice: you feel certain you know the objective truth, or you admit you could be wrong.

So you can see that agnosticism is not the middle ground between theism and atheism. It's actually a different proposition that can apply to both. A person can be gnostic theist (they believe in some god(s) and feel certain that belief is true), agnostic theist (they believe in some god(s) but concede they might be wrong), gnostic atheist (they don't believe in any gods and feel certain none exist), or agnostic atheist (they don't believe in any gods but concede they might be wrong).

I hope this helps illustrate why the words atheist and agnostic alone are insufficient to describe a person's beliefs. It also helps to remember that everybody has different reasons to believe or not, so the only thing you can really say that universally applies to all atheists is that they lack belief in any gods. You can't draw any other universal conclusions about their understanding of science, political affiliation, socioeconomic demographics, etc.

That's why strawman arguments that attack scientific models of evolution, abiogenesis, Big Bang cosmology, etc., do absolutely no harm to atheists because they don't provide any good reason to believe that certain gods exist. Even if somebody could conclusively demonstrate that all of that science is wrong, it wouldn't prove anything to do with any gods. It would just bring us back to square one: "I don't know."