The main goal of migrating to MariaDB is not performance driven. More so,
I think it's in WMF's and the open source communities interest to coalesce
around the MariaDB Foundation as the best route to ensuring a truly open
and well supported future for mysql derived database technology.
Performance gains along the way are icing on the cake.
My guess is that they have been using MySQL for a long, long time (possibly since the beginning)m, and see no need to migrate to something different. MariaDB is API-compatible with MySQL, so this migration will not involve much changes at the client side; it's mostly thorough testing that needs to be done.
Note that unless your mysql server has more than 4 (physical) cores, you aren't going to see any performance improvements from any variation/fork of mysql. 8 cores is where things start to diverge under load.
They aren't starting from scratch. Migrating your database is a complex and risky project, and not the kind of thing you do unless you have a very clear, very pressing reason, like horrible performance and stability issues, or saving tens of millions of dollars in licenses. Migrating to MariaDB is more like upgrading a minor version, with minimal risk and zero downtime. And they have a lot of experience in MySQL, almost all of it transferable to MariaDB.
I use MariaDB on my Mac for most things that require MySQL. It's a drop in replacement for most things. Is there a special edge case with media wiki they had to solve or is it now just formally announced as being supported?
So, it's pretty much a drop-in replacement. But as with everything, time m ust be spent testing and so on before rolling it out to production. I believe a transition to MariaDB has been planned for a while now.