The assumption that one is claiming certainty is also made about atheists generally, not just about strong atheists. It's common for atheists to hear from apologists that it's not possible to be absolutely certain that no gods exist anywhere in the universe, therefore the position of atheism is illogical. This both misdefines atheism and misrepresents the nature of empirical claims.
The double standard here is easy enough to see: theists routinely claim that some sort of god exists, but no one expect them to be able to support a claim of absolute certainty. Theists are allowed to be less than 100% certain about their claim that some sort of god exists, so why can't atheists or strong atheists be less than 100% certain about a claim that some sort of god doesn't exist or that no gods exist?
Rationally justified beliefs can be, and often are, based upon evidence which falls short of absolute certainty. We aren’t absolutely certain that the sun will rise tomorrow or that our brakes will work the next time we try to stop our car; nevertheless, we believe these things because we have ample reason to do so. We do not need to be omniscient or omnipresent to hold such beliefs, and the same can be true for the belief that no god exists.
