Like he clearly was referencing Trophic Thunder’s “Never go full retard” line.

I don't think that is a clear reference; I think you're giving him undue credit. Saying something is "fully " surely predates the movie. I've seen Musk's failures more in the timing than the method: he fails because he doesn't know when to meme (and when not to) rather than a geriatric ignorance on how to form the meme. So he should have been able to make a closer, more apparent reference, like "You went full retard," instead of what he did there.

I think he is just being an asshole.