Good for them. Both MySQL and PostgreSQL are Good^. But supporting both doesn't work and it totally makes sense to focus on one. Personally, I don't generally use PGSQL but my GitLab instance does. Why? It's easy to run a dedicated PGSQL instance for GitLab using Docker. Or... Using AWS RDS. Or... Any number of ways.

(^ Good is relative. People have their opinions on which is subjectively better but at the end of the day, lots of big sites and applications use both options to great success.)