IPO's depend on premium pricing and structure. If he were to follow the Google dutch-auction approach, not oversubscribe in any way, and limit subscription while addressing in the filings the wildass nature of this "venture", he'll probably pull it off.
Investing is not strictly logical - more emotional, and Space-X definitely provokes those. Elon's chutzpah is legendary. He doesn't have a history of ritually lying to himself or other investors, so he gets give a large benefit of the doubt.
More likely this is deal posturing as a way to twist arms of other investors/partners you aren't aware of.