Their Github indicates there's cross-pollination:

> Moreover, it includes patches from Percona, WebScaleSQL, and MariaDB.

Getting out of the business of having their own?

Looking at their commit history, it's not a fork in the sense of "we're actively developing our own new features and diverging from upstream", it's a fork in the sense of "we've taken a stable release from upstream, applied some of our own patches, some backported patches, and some patches from other forks."

It's a fork as much as the stable packages that RedHat or Debian ships are "forks" since they have a few patches applied to customize for their environment and fix a few issues that hadn't made it upstream by the time they picked a stable release to base on.

MariaDB is an active, independently developed project. They are probably funding MariaDB because working with upstream developers of an actively developed fork can be more efficient than striking out on their own with another actual fork. I wouldn't be surprised if they eventually start using MariaDB, and either rebase their AliSQL patches on top of it or drop AliSQL altogether (much of it just looks like backports of bugs or features).