I think it is indefensible. The question regarding the origin of the universe is indeterminate. To say outrightly that there is no creator seems about as bad as saying there is one, though not as bad as saying there is a personal creator that has spoken to man and given them commandments and what not.
There are lots of creators. Chris Carter is the creator of The X-Files. Everybody on TFT is the creator of at least one post. That doesn't make us all gods. But you mean that to say outrightly that there is no creator
of the universe seems about as bad as saying there is one. Well, that depends. What does "universe" mean? Dictionaries disagree with one another.
"the whole body of things and phenomena observed or postulated" -- Merriam-Webster
"All existing matter and space considered as a whole" -- OED
So what do
you mean by "universe" when you argue that to say outrightly that there is no creator of the universe seems about as bad as saying there is one?
If you agree with Merriam-Webster's definition, then your argument is plainly wrong. Of course there is no creator of the whole body of things and phenomena observed or postulated. To postulate such a creator is to postulate a thing that created itself, which is self-contradictory. Strong atheism about that sort of god is perfectly defensible; it's the only defensible position.
If you agree with the OED's definition, then perhaps it's true that it's indefensible to say outrightly that there is no creator of all existing matter and space considered as a whole. But strong atheism is saying outrightly that there's no
god.