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.
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.
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.
Allow non-Safari browsers (not just re-skins!) on iOS and I'll believe the author.