I wouldn't assume there is a "YC tech stack". From individual blog posts I've read there is great diversity among YC companies' tech stacks. I also wouldn't assume that rails + angular is, or is even considered to be, the cutting edge of web stacks.
Maybe. On the other hand, Paul Graham has strong views on what great hackers are like, which are likely to color his evaluations of candidate companies. It would be surprising to see a YC company running an Azure/Windows/C#/Visual Studio/TFS stack. And there are good reasons why someone might do so.
I believe Loopt was on C# stack and its founder is currently leading YC, so I'm sure Paul didn't take stack too seriously when evaluating companies. There are a lot of good reasons to use Microsoft stack, and I suspect more than a handful YC companies use it.
Paul said it is advisable to not use enterprise technologies if you're willing to attract high quality engineers only. He never said it can't be done and it is never implied that high quality small number of workforce is important to every business. If you're in consulting, for example, you'd want a larger workforce you can get for cheap. But I think you already mentioned that point, just throwing it out there.
