These are very desirable jobs. People would bend over backwards to work at Twitter. Younger, cheaper people who have strong ideas of how to fix Twitter and would actually try to push these ideas. And these seats are currently filled with older, expensive employees who like things the way they are averse to things breaking. Either that or you are a rough and tumble old timer who can thrive in chaos.
This is very intentionally the model at his other companies. Or places like Amazon where they have almost perfected the art of extracting exactly 3 years of valuable work from young developers.
I don't think Musk actually cares about that 84th hour of work. But I think he is very much trying to signal to current and potential employees that this will not be a comfortable long term career prospect.