How is Musk an engineer? Musk has a degree in physics and economics according to Wikipedia. At least where I'm from, you don't call yourself or someone else an engineer if you didn't go to engineering school.
What Musk was doing 25 years ago when he was at university isn't important any more. Are we supposed to just ignore the 25 years of experience he's gained since?
It's very simple: if he does engineering then he is an engineer, regardless of what degree he holds.
Does he, though? He's CEO if 4 companies IIRC. I would be very surprised if that leaves you any time at all to do stuff that qualifies as engineering (in the traditional sense; not in the bullshit uptitling sense; "software engineer" is only one step from "ninja rockstar").