As someone who has tangentially interacted with YC, I seriously doubt that is the reason. YC cares about the team above all, then followed by customers/users/traction (but really, it's the team). The addressable market is not very high on their list of concerns. Though it certainly is a factor, I don't think it is the reason for rejection.

One question I have is how well does the team know each other? What are some of your team members' accomplishments? Honestly YC doesn't care very much about all the degrees you have (I would even argue that the fact that there are 5 degrees among the 3 of you is a negative signal for YC because it indicates credentialism, but that's just my opinion).