That sounds like the petty, vindictive, childish type of stunt we've all grown to expect from him. That's what's making this so hard to parse out, 2 rich assholes with a history of lying are lobbing accusations at each other. They're both wrong, and maybe both right? But it's so messy because one is a colossal douche and the other is less of a douche.
Thing to keep in mind. That Musk even might force to open up GPT4.
That would be nice outcome, regardless of original intention. (Revenge or charity)
Edit: after a but of thinking, more realistically, threat to open sourcing gpt4 is a leverage, that musk will use for other purposes (e.g. Shares in for profit part)