I would argue that the render engine of web (CSS) is much more sufisticated than Apple's and Android's UI component frameworks.
A well written single page fully clientside rendered web app can be as fast and good-looking as a native app. In my opinion it's easier to develop as well.
I've done a lot of web development and iOS development (not so much Android so can't comment too much). I don't really agree with most of this statement at all.
The web is a powerful platform for a lot of reasons but I don't agree that it's easy to develop for, iOS is a much less hostile and predicatble environment to run client side code in. I also don't believe that CSS/HTML are particularly well suited to rich mobile applications in comparison to UIKit or SwiftUI. I still write PWAs and native apps and don't see why they can't coexist. Proponents of the PWA approach seem to really want PWAs to replace native development.
There is a lot of functionality and APIs that native apps have access to on both platforms, I'm not sure I see the point in browser developers implementing every single one of them when a subset can cover 70% of JSON-viewer type app needs, the remaining niche can be written as native apps.
