Apple doesn't allow 3rd party engines on iOS because if they let web apps get too capable then people will stop paying a 30% premium for App Store apps.
Apple is quite fine with people making web apps, and have been expanding Safari's capabilities to support them. In the latest major iOS, PWAs can now even send push notifications to the user.
Partly because for a long time Android allowed apps to do their own billing for subscriptions and other IAP, as long as the content you purchased could be accessed on other platforms (with a big carveout for games, which always had to use Play Store billing for IAP). That's why Spotify railed against Apple for years but didn't mention Google.
They changed their rules a couple years ago to make it pretty much consistent with Apple's policies, but I think some of the bigger players got exceptions.