Currently, as it stands right now, in the state of development it is in, mySQL is *required* for greatest compatability. I'm not saying you're wrong, just that Postgre, MSSQL, sqlite, or any other SQL server isn't completely supported. The only reason we have limited support on other systems is that people have ported them.
Yes, the wording should change, but right now, mySQL is the most compatible one, and I believe for that reason, should be the one we try to urge people to use (although not required).