I don’t know if it’s really THAT big of a leap of logic for him to say that after “temporarily” breaking through to the 7 Star realm that he is as strong as someone in the 9 Star realm.
Earlier he learned that his own body/soul is unique in that it has a similar state to what the bodies/souls of summoned beasts are like.
So when he does crazy stuff like bringing back the Moon in its primordial form it doesn’t just give his various beasts a power boost. It also gives him a power boost directly, thereby increasing his own stats.
So while he might have just temporarily broken through to the 7 Star level, with the boost of the Moon he gets buffed to the point of where the 9 Star level would be at.
