I'm curious what YCombinator has to say about that kind of behaviour. Is this a unique case? Has it happened before? How do you prevent it from happening? What's the benefit of being able to say you're a YC founder?
Being able to attach the YC brand to your resume is probably very helpful when seeking investment for a new venture or applying for a job at a BigCo.
Although I have no knowledge of the situation beyond this (very well written) post, I don't think it's farfetched that a couple developers would join an existing company just to experience YC and improve their Valley credentials.
Why do you say so? Surely that's just one signal among many, and not an absolute indicator of anything?
As for indicators, I think that people who have worked together for a shorter amount of time are more likely to break up. This is because fights and disagreements between co-founders are inevitable, and in startups they are more intense compared to regular business working relationships. When you have history with your cofounder, then you've likely survived those fights and are more likely to get through the ones coming.
Of course, this is simply one of many factors.