I imagine by "default DB" what they mean is the default that a package recommends and is configured for.
So in a situation where Drupal, Django and Wordpress can all use either MySQL/MariaDB/PostgreSQL/($AlternativeDB), the package itself will recommend MySQL, and the config files will probably reference MySQL (if such a reference is required).
You're welcome to pick an alternative, but there might be a bit more work involved...
Having tried to set up a couple websites using a CMS - admittedly on Debian/Ubuntu - I have to say that MySQL is pretty much the only default.
I would really, really, really like the default to be PostgreSQL. Both for ideological reasons and for pragmatic reasons. But the simple fact is that the developers of many systems might say that non-MySQL DBs are supported, but they onlyt seem to test with MySQL.
So unless you want to be the tester, you pretty much just pick MySQL and go with it.
My brief experience with alternate DBs for what will probably be the most popular usage case therefore leaves me unconvinced that this is a decision that Fedora should be taking.
This decision should be being made upstream with the projects themselves. Let them take the decision on what's their recommended DB, and let them do the testing for the users. Anything else is just asking for trouble.