That is basically the opposite. Being ordered by your boss to go pull an all-nighter at his other company for no additional pay, as opposed to voluntarily taking an additional job along with the associated salary because you were bored and felt like it.
Whats to say those engineers aren't taking PTO time or a leave of absence and getting paid a consulting rate by Elon/Twitter?
"Other times, two Tesla employees told CNBC, Tesla workers are pressured to help with projects at his other companies for no additional pay because it’s good for their careers, or because the work is seen as helping with a related party transaction or project."