I can say that because it's literally not true. Omnipotent literally means all powerful without limit. If a character is stated to be able to lift 100 tons but then the author decides that they fail to do so, then that's that. Actions speak louder than words and if the author says one thing but contradicts it with his own actions which are at least the same value, then it is a contradiction. There's no arguing that as its a fact. Alien X is not omnipotent. His high showings and low showing are supersceded by any number of characters not labelled as omnipotent. The author claiming omnipotence in his own series and contradicts it in his own series is one thing. But in another series where that author's word and actions are king, he or she doesn't have to cater to the faults of the other author. If the creator of Battler Ushiromiya gave him greater power than the other author's interpretation of omnipotence, then that's the way it is. Within the Ben 10 verse that's fine. But Other series do not have to tolerate the flaws of another.
Like I said, the author is the "true omnipotent" he could do whatever the heck he wants and he'd know for a fact that it's true. If he decides that the character couldn't liftt it then that feat is fake. Real life is above fiction afterall. Writers > their characters. Omnipotence has always been above non omnipotents, therefore alien x wins tthis uunless we go by feats only. Since this battle is not held in either of the creator's verse but in a neutral one then the "mistakes" don't apply. It's not a "mistake", the characters wouldn't call it one if they could talk, since a true ommipotent did it.