Their rationale for not allowing other browser engines is that allowing JIT (writable AND executable memory pages, in general) would make the hell break loose.

This is an insult to their OS team. macOS appears to be working just fine.

Well, if there is an RCE vulnerability, that would allow the user to run arbitrary code on their own device with the same permissions that the browser process has, which is clearly unacceptable in the iOS world.

I don't see how that could be acceptable in macOS world.

On macOS you as the user already have complete and unrestricted access to your computer. You don't jailbreak your mac because it already comes with root and an unlocked bootloader. Mac apps also have much more access to the underlying system than iOS apps do, and this helps with better sandboxing, I presume.

This was a good argument in the past, because there were very few ios exploits and apple was proactive in hunting them down, to prevent jailbreaking. Not sure if it is a valid argument today.

JIT is already allowed on iOS since 14.2 though.

For externally-loaded or user-supplied code as well?