Yes Knex supports MySQL but the issue is often in the details. Sooner or later small differences between the DBMS end up biting us and one release might work with Postgres, because it would have been tested with it, but break with MySQL, or introduce things like data being silently cropped and so on. So the only reasonable way to support two RDBMS is to have automated tests for both, and that's hard to do.
You're free to use MySQL anyway, it's just a config variable to set, but for now that won't be officially supported.