This has to do with the definition of the word “omnipotent”.
I think we can all agree that the meaning of the word “omnipotent” can be captured by the statement “being able to do anything”.
The question is: what does the word “anything” include?
Does the word “anything” include things such as “the creation of square-circles”? If so, nothing can be omnipotent, because nothing can “create square-circles”, given that the term “square-circles”, being a contradiction, has no meaning.
Similarly, one can ask, does the word “anything” include “the creation of things that are not caused?” If so, nothing can be omnipotent, because nothing can “create things that are not caused”, given that the statement “to create something that is not caused” means “to cause an uncaused thing”, which is a contradiction in terms and therefore without a meaning.
Is that how John defines the word “omnipotent”?
I think he clarified that what he means by the word is “being able to do anything that can be represented by a non-contradictory statement”. (Not his words but I think that’s pretty much what he means when he says “being able to do anything that is logically possible”.)
Define it the way you’re defining it and the word becomes meaningless.
EDIT:
Let’s put it another way.
The reason I can’t create square-circles is not because I am incapable but because the term “square-circles” is a contradictory one, and therefore, meaningless.
Similarly, the reason I can’t create things that are uncaused is not because I am incapable but because the statement “the creation of things that are uncaused” is a contradictory one, and therefore, meaningless.
If I say “Do lnmdlasudoiq!” you wouldn’t be able to do that thing, not because you’re incapable, but quite simply because I mean NOTHING when I say “lnmdlasudoiq”.