That's not very precise. Legally, Apple doesn't allow to use iOS SDK, i.e. to build such applications. If you manage somehow to build it any other way (not using Apple's SDK), this restriction has no meaning, and while Apple could reject such kind of software from their app store, you could deploy it to alternatives, like Cydia, and still be legal, since you didn't violate any licenses.
I.e. for browsers makers the concern is not that they'll be rejected from the app store, but that they can't even legally build them with the Apple's SDK.
The rule above is not from the App Store guidelines, but from the SDK license. I.e. even if you use alternative app store without restrictions (Cydia), but build a browser with Apple iOS SDK - it's still illegal, and that's why major browsers like Firefox wouldn't do it.
I'd say it's a very ugly practice, to use development tools license to ban competition. It's like saying buying this chisel you are bound to only carve logos of the chisel making company.
I.e. for browsers makers the concern is not that they'll be rejected from the app store, but that they can't even legally build them with the Apple's SDK.