While this is technically true, WKWebView (which I believe is used by all non-Apple browsers on iOS) does allow Javascript JIT, because the renderer runs in a subprocess with these permissions. See: https://news.ycombinator.com/item?id=19379739 .

But there are many other reasons besides JIT to want to have non-Apple-Webkit-based rendering engines (including wanting different JS engines with their own JIT) - and so IMO it's very much a restriction that regulators should force Apple to relax. The security considerations should be no different than those on a desktop platform.