I think that motto refers to features more than underlying technologies.

Correct, see our see our ‘boring solutions’ subvalue https://about.gitlab.com/handbook/values/#boring-solutions

I agree, but if you’re a MySQL shop that’s a feature.

I guess I'm in a MySQL shop, but when it comes to Gitlab, I just want it to run its regular upgrades without me having to do anything (and it does! I love Gitlab). I never considered running Gitlab on MySQL.

We rarely have to fiddle in the database itself (it may have happened once or twice during our migration from Redmine), for the rest we use the Gitlab API.

If you are at the level where you need strong control on your Gitlab install, then you're a Gitlab shop, and you should either use the recommended dependencies, or assume that you will be helping maintain the niche features.