Either way -- to say that C# would be a better platform for developing an open source project is far-fetched at least (the developments of the last year notwithstanding, although agreed that MS has come light years in that direction from where it was just a short while ago), and to say node would be better is, well, not uncontroversial.
I understand, given your position, this is a point of view you must advocate (although...node? really?), but neither point makes Jeff's rationale anywhere near "all wrong."
Agreed re: SQL Server vs., say, postgres. I'm actually not sure why we don't see that option (ASP MVC + postgres/mysql) a lot more often than we do.