Well, look at it properly, the bug is about optimization of a query that does not make much sense. Sure, it could be done better, but why would you issue such query at all.
If you look at problems that Oracle/MySQL engineering tackled, they are somewhat different - data compression, online DDL, parallel replication, GTIDs, InnoDB scalability, etc - these were huge efforts and get reasonable focus. Think of all the bugs that were not filed against MariaDB...
Count InnoDB engineers working for Oracle and for Maria, unfortunately that will not be balanced. Even Percona's InnoDB expert Yasufumi Kinoshita ended up working for Oracle lately.
Sure, Maria can do all sorts of tricks in SQL-land, but it is not the full picture. Oracle has much more engineering power dedicated to supporting MySQL, and they also have customers who are doing bug escalations as well.
Disclaimer: I used to work at MySQL AB and currently am working on a deployment that builds upon Oracle's MySQL tree, see https://www.facebook.com/MySQLatFacebook