It's true he could write off xAI today and the company could still fetch a trillion-dollar valuation. But I was more referring to his stated intentions - between his stated plans, his actions taking SpaceX from a profitable company to spending basically all their revenue (plus a rumored large chunk of what's raised via its IPO) on AI, and seeing his tendency to make bet-the-farm bets on Tesla, I think it's fair to say he's committing to bet all of SpaceX on xAI.
SpaceX has invested a small amount as a share of its value in XAI, and could survive the loss of its investment.