This has nothing to do with why they don't want game streaming apps on the store. Running a Gecko shell instead of webkit on iOS wouldn't change their 'moderation' (preventing other app stores from being in the Apple App Store) stance.

I know this, but as judge2020 pointed out, that's irrelevant to my point. That said, it is relevant to the whole anti-trust thing because Apple has a stranglehold on browser features (such as PWA stuff).

It is very relevant. Another browser with e.g. push notifications in the appstore would be the equivalent of stadia displacing appstore games - but for 99% of all of appstore.