It's normally OK for employees to do volunteer work out of hours. I used to do it myself for an animal shelter in the UK at weekends, my company knew about it and had no problems at all. I didn't do it instead of sleeping either, I managed to work the day job, volunteer, and have a social life with both my colleagues and other volunteers as well - amazing huh ????.
It might even be illegal for Musk to stop his staff doing volunteer work outside their contracted hours.
As long as they aren't doing it for a Tesla competitor then I don't see why it would be an issue.
Clutching at straws.