Merry Christmas to you, too!
Brian Kelly has shortcomings. But he has also coordinated offenses for years. Rees' dad also has extensive football coaching experience.
If we were talking about a young OC who played any position other than QB, I would agree with you. If after playing all the football Rees played, combined with having his dad as a longtime resource, and with five years' coaching experience and intelligence, if he isn't qualified to be the OC at ND by now, he's not cut out for coaching.
I'm not saying he's the best OC or that he won't get better through experience. But I also think he's far from unqualified or is in over his head.