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.