Hello and welcome to the community.
- php might not be the current hype, it has been around for a pretty long time and makes up a decent percentage of active websites to date. Also it's a tech the founders have most experience with, switching language would be very costly.
- Flarum is meant to be extensible. As an extension dev turned core dev I must confess I'm hammering hard on keeping extensibility a fundamental element of our app. Both the api and the SPA are currently pretty extensible so please clarify what problems you ran into.
Although I truly believe you mean well your title isn't really too friendly. Also, sorry to say, using your title "full stack developer" as closing argument never works well when you list only backend languages.
You know, every once in a while, someone will show up proclaiming the choices made are a "big mistake". We should be using react, angular, ruby, go ..
In the end everyone has the right to an opinion including the owner of the project.