It is certainly an impressive feat, they all work and perform pretty well. They're doing this to dog food their own engine, mainly the UI side of it.

As far as I am aware all apps now use this instead of the native/web stacks they were using before, but its been a bit of a rocky transition.

Having used their website for years it just works and is pretty fast (However it is much slower in recent years than it once was), the new in-engine app just feels kinda clunky, its all incredibly slow and janky. Definitely not the smoothest of transitions