So? If the app's JS performance is fast enough to play the game, then there's no problem if it would be better on a desktop browser.

(Or do you mean the iOS browser? Still, doesn't matter. If you want to sell an iOS, there's just no comparison between the monetization facility of the App Store compared to the browser).

Depends: maybe they used something like Ejecta (https://github.com/phoboslab/Ejecta) which doesn't use the standard webview but re-implements canvas on top of opengl.