The problem with lumping in agnostics with atheists is that you presume all agnostics are atheists.
Who is lumping the groups? Were actually being careful with the terms and using them as they were intended, and in a way that is consistent with how they have been taught in philosophy classes. Again, you seem to have a problem with nuance.
We have shown the root meaning of the words.
Theism - Belief in a deity(s) (the·ism n. Belief in the existence of a god or gods)
Atheism - Without belief in a deity
Gnostic - with knowledge (Gnos·tic adj. Of, relating to, or possessing intellectual or spiritual knowledge.)
Agnostic -without knowledge
a-, (Greek: prefix; no, absence of, without, lack of; not)
We have also shown that atheism was defined to mean without belief long before the term agnosticism was invented, making it impossible for atheism to hijack the meaning.
In response to this you choose to double down on ignorance and support it with this line:
They are not. Hence, you are wrong.
Because agnosticism pertains to knowledge, while atheism pertains to belief, they are not mutually exclusive. Beliefs are a spectrum. There is nothing stopping someone from saying "I have absolutely no way to prove God, yet I have no doubt in my heart that he exists". This is an agnostic theist. You can be without knowledge and still be with belief, thanks to faith. If you think you can know god and in fact do because of the bible or because of miracles, you are a gnostic theist. You have knowledge and belief. Likewise you can be with knowledge and not with belief, a gnostic atheist, which is a position I have never seen anyone take, but is possible according to the mechanics of the terms. An atheist who claims to know that there is no god is making a leap of faith as well. The most common type of atheist is an agnostic atheist, one who says 'I have no knowledge of God, therefore I have no belief of God'.
Anyway, I don't like arguing definitions. Especially well established ones.
There is no arguing definitions of precise terms, which is why you have been unable to provide any support for your use of atheism as exclusively meaning a positive claim. Indeed, you seem quite happy to perpetuate your misunderstanding, perhaps because it puts you in the position to make comments such as:
You can cease and desist with your attempt to hijack a term that doesn't belong to atheists or theists exclusively now.
Which incorporates yet another strawman. None one here has said agnosticism 'belongs' to any group. You are free to go argue against those immature and bitter people who claim to be able to prove God isn't real, but you'll have to do it somewhere else, no one has made that claim here in the 3 years i've been part of the community.