A- means lack of
Anti- means opposed to.
I actually thought the same thing.
It turns out that agnosticism makes an assertion. An assertion that some things are unknowable.
Athiest don't know if some things are unknowable, they just lack the assertions of faith.
EDIT: Oh! I see, this is a new kind of wording convention. "hardline atheists" does pretty much mean antithiest. My bad.