i'm sure viz will clarify this but right now i'm hella shocked that kawaki only expressed the possibility of naruto being stronger than jigen. naruto is rikudo blessed what has jigen going on for him?
Are you forgetting that Kawaki is a kid and he is also not from Konoha, so he probably doesn't even know much (if anything) about the bijuus or the power of the SO6P?
How would he be expected to truly know how strong Naruto is? The word "might" makes sense when you look at it from Kawaki's perspective.
Plus, Kawaki probably hasn't seen Jigen's full power either, so the word "might" suffices even more.
Lastly, we know nothing of Jigen's abilities right now, so who are we to say that Jigen couldn't be stronger than Naruto? Heck, we haven't even confirmed yet whether or not that diamond on his chin is a karma seal, let alone any of his other jutsus.
You are making a typical fan error. Fans have a bad habbit of seeing a really strong character like Naruto, and then assuming that this character's power is the limit of the verse, and that no subsequent character can be stronger.
Then, when a character does come and surpass the character in question, fans scream and shout "asspull bad writing", no matter how much foreshadowing, hype or logic/explanation was given.