I may be rude, but Elon Musk is just a 22nd century snake oil salesman.
OTOH how would you go about assessing his engineering ability? He was accepted into a Stanford PhD program. An ex-employee of spacex was amazed how he absorbed a textbook of rocket science.
Unlike Eistein, he hasn't written any papers himself (AFAIK), so we can't assess his work directly. I would guess his primary role is being a technically and business-aware coach for engineers who actually do the work - like Steve Jobs. But maybe he is a genuine engineering genius himself - but how could we tell?