Oh, it's PHP - neat. Personally I think Statamic is the entry to beat for flat-file CMSes in the PHP space. This project is the Symfony flavor I suppose (vs Statamic being based on Laravel).
Personally think https://getkirby.com is the entry to beat but I guess it’s just because I’m used to it and it works incredibly well for my use case.
https://devboard.gitsense.com/getkirby/kirby
https://devboard.gitsense.com/getgrav/grav
Historically speaking, grav is more popular as it had over 2000 contributors, compared to kirby which is about 500.
Full Disclosure: This is my tool