He's inaccurate for common use, and common use is what defines a language. If the terms "gnostic atheist", "agnostic atheist", "gnostic theist" and "agnostic theist" gain ground in the vernacular, then he'll be correct. Until then, your definitions of atheist and agnostic are correct.

In common use, agnostics are people who don't believe or disbelieve in a god, and atheists are people who don't believe there's a god. They're both belief systems... if differing opinions on the nature of evidence can be counted as a belief system.

Atheists believe that without evidence for a god, one can conclude that god doesn't exist.

Agnostics believe that without evidence for a god, one can conclude that the existence of god is unknowable.

They're both viable and intellectually defensible systems, provided that the person holding those beliefs is consistent. To be consistent, atheists need to disbelieve in all gods, and agnostics need to be agnostic about all gods. If an agnostic says he's sure that Thor doesn't exist but thinks the Christian God is unknowable, they're not an agnostic at all. For the same reason that an atheist who believes in one God is a theist.