I don't get how this could be a coup, Trump was duly elected, and he's delegated this power to Musk. It could certainly be bizarre and highly illegal, but to me, the essential piece of a coup is unseating the rightful leadership, and there's no element of that at present.
Judging from his last term, at some point Trump is likely to get tired of Musk, kick him out of the administration, declare he always thought Musk was a bad guy, and pretend like he never listened to him. If Musk tries to stay in after that, it could be a coup.