Naaah pretty much every modern app has a good chunk of platform agnostic HTML5 in there, most app UI's are HTML5 based at their route, abstraction is a potent tool especially for front-end development that is useful for dev teams big or small. Abstraction doesn't have to be messy or slow, you can write an interface in HTML5 that works on basically any platform but still feels smooth on a 32-bit A7 ARM processor.
Steams traditional interface completely bypasses Window's compositor & uses its own badly made one which is why it could be so buggy(Especially on assymetric multi-monitor setups, where the interface literally hasn't been usable for like the past decade if you have one screen in portrait), not sure how this slow revamp is changing things but in many ways Steam's problems were from the opposite of abstraction. There are definitely far too many layers at play though, it's like a house that's been falling down for a decade but the landlord just gives it a new lick of paint and some botchy repair jobs each time and says it's ok. It's technical debt taken to an extreme.