My guess would be stuff like upcoming products, business connections, contracts way of doing business etc.

However I doubt that Musk has any grounds for claiming that "the most useless employees" who weren't even treated with dignity upon firing them would have transferred those to Meta illegally.

IMHO this is just Musk freaking out that the next time Twitter doesn't work as expected or asks for money, people would have real alternative to move on.

Reddit needs something similar.