Are you seriously asking if YC would have accepted the former co-founder of a company that sold for over a billion dollars? Isn't the answer to that question obvious?
And so no, the answer is not obvious.
Please don't, that is a completely different question. I'm honestly not sure how you managed to conflate the two.
Granting 50% of equity is a lot, and as an experienced entrepreneur, Kyle would know that. But he wrote it down as the plan. Why?
These are the sorts of questions a court will be asked to ponder.
Given Kyle's background... The answer is yes.
Yes, without a doubt. He's previously YC by way of Justin.tv / Socialcam / Twitch. If you're good enough to go through YC once and have a track record of success you're a shoe-in for your next venture.
Realistically Kyle would have been accepted to YC without any idea and he would have been wildly successful with Cruise even without YC.
Put another way, if Jeremy didn't contribute anything, then why did Kyle start working with him in the first place? Put him in the YC video? List a planned 50/50 equity split in the YC application?
These seem to be at least reasonable questions IMO.
All that said, I think the issue here is that Sam Altman personally invested in Cruise, which creates a subtle conflict of interest. It's in Sam's best interest as a Cruise director to crush Jeremy, but perhaps (as the parent suggests) it would be in YC's best interest to just stay out of it (above the fray).