So apart from the ad blocker, that's ... features, smoother, better. What?

Edit: I'm not trying to be rude (it comes naturally). But you just explained "great" as "better, with more". I guess smooth might mean faster, which might be because it isn't doing ads and tracking. It seems to come back to third-party being the crucial difference, and "app" not mattering.

UI/UX is not that tricky. Caring about your users is the hard part usually.

Third party clients could be webapps, too, of course.

Apps can bypass websecurity (CORS fetch), that allows for third party clients for example on video platforms using their internal APIs.

I don't think the reddit clients work this way though.