Also tooling focused on their dev flow.
In difference to the article implying that they use out-date project tooling they don't(). They VCS isn't out-date but in some way more modern then git, it's just focused on dev flows similar to theirs to a point where using typical git dev flows will not work well. Similar not using GitHub is a more then right decisions for how the project is manage, github is too much focused on open contribution projects.
(): You could argue they use "not modern" tools like C and similar to do modern things like fuzz testing. But the articles author clearly puts a focus on the project tooling highlighting git/GitHub so reading it as implying that their VCS is "not modern" i.e. outdated i.e. bad seem very reasonable IMHO.
As I see it, the quality comes from the fixed need, tight scope and small professional team
Fixed need -> Somebody always needs a database
Smaller scope -> Less features, less code, less bugs
Professional team -> No newbies around to break things
Now, to be sure, this kind of development model can only work for some projects.