In the absence of documentation of how the equity dissipates, it would be assumed not to. In that case, I'm not sure it's dismissable as a technicality: if an early, vital partner has to leave, you have to weigh the costs of buying out their equity stake against current cash demands. When the future of the company is tenuous, the value is small and Guillory surely would have accepted a song for a buyout, just as Walter White took a $5000 buyout for his huge stake in Grey Matter (or the Apple guy); but they may also need that money very much.
If you take the approach of "well [in hindsight] OBVIOUSLY they didn't mean to leave him with that equity", you're effectively giving founders a free option on the shares of silent partners, unfair in its own way.
They applied together to YC. They got accepted, but Guillory didn't go. Vogt said he didn't want to work together anymore. A business entity was later formed, in which Guillory had zero ownership of because he left the project before it even began and had nothing to do with it. The only basis for Guillory here is that they did at one point agree to work together.
This is like me saying to you, "Hey, do you want to go halfsies on a lottery ticket?" and then you responded, "Yes". Later, I tell you I no longer want to go halfsies, then go out to buy a ticket by myself and win. You then later sue me saying you get half of the winnings because we had previously agreed to go halfsies, despite the agreement later being revoked and you having never contributed and never planning to contribute anything to the cost of the ticket.
It isn't an issue of assuming a vesting schedule, it's an issue of someone deceitfully claiming ownership of something they never owned.
Seems like YC is asking people to hold to that structure when it is in their interests, but ignore it when it is not.
To be clear, don't think this is terrible. Just, if you are going to claim the split means nothing, then why force applicants to create one in the first place?