The chofetz chaim zatzal has a beautiful vort on this that does not explain why they said their father was not involved with korach but does explain why moshe had to ask hashem and did not answer himself.
He says that the reason he asked hashem is not because it was too difficult for him to answer, but that once they had said ???? ?? ??? ???? ???? ??????? ?? ? ???? ???, then ??? felt he can no longer judge their case as that could be considered a bribe, ????. Our father was not with ??? ‘s group against you but was on your side.
So he had to go ask hashem got a teshuva!
But the question is a great question and SaysMe I like your answer!