Elon spends nearly as much time at SpaceX as he does Tesla. There's no question that Elon mucks in at the lowest levels of engineering at SpaceX.

If you want evidence from a reasonably neutral observer, take Sandy Munro (himself an engineer who has worked on everything from cars to aeroplanes). He recently interviewed Elon, ostensibly about Tesla but the interview was in a meeting room at SpaceX. After the interview he was invited to a two hour design review meeting and was "blown away" at Elon's depth of involvement.

https://youtu.be/S1nc_chrNQk?t=370 (6:10 to 8:45)