Yeah, that really stood out. Seven out of 8 or 9 used MySQL.
It's not so suprising. You don't really need the overhead PostgreSQL brings in for most web applications. You probably don't need the overhead of SQL Server, either.
This sort of stuff is irrelevant. These sites are not successful because of the backend.
I could be wrong, but I don't think anyone's implying that these sites' success is due to their backends. However, backend choices do affect uptime, scalability, ease of development, ease of finding/fixing bugs, etc. So I would certainly take issue with your claim that "this sort of stuff is irrelevant".