That sounds very much like the distros vs. upstream argument on bundled libraries... for exactly the same reason, unbundled system libs allow the distro/OS to patch all users of that lib with a single patch to the system lib, instead of requiring dozens of apps, some of them obscure and rarely used enough to not be as well tracked as the major browsers, be patched.
Seems quite reasonable to me.
Now if, say, firefox isn't allowed to run on iOS (I don't know, iOS is a walled garden I don't even visit), or links or lynx, the text-mode-browsers, because they use some non-webkit library, then that's a different matter indeed. But requiring unbundled system libraries is standard practice among distros as well... for good reason!
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