Theist version: An atheist is certain there is no God, an agnostic is not certain.

Atheist version: An atheist believes there is no God, an agnostic doesn't know.

I think (believe?) that even this author has it the wrong way round. Rather than say "An atheist believes there is no God", I'd put it that "An atheist doesn't believe there is a God"

There's a subtle difference. To me, the second one implies that there is a lack of belief in something, rather than a positive belief in something (in this case, "the lack of a god"). This leaves open the potential to be both agnostic and atheist. I can't say definitely that I know there is no god, but I certainly don't believe in one!

I'm just trying to be logically consistent, although, by my definitions, a theist should also logically be an agnostic, as they don't know, but they do believe.

If asked, I'll always say I'm an atheist, but if I get into a discussion, I'll add that technically I'm an agnostic as well.