The fact that these browsers exist is very pertinent to the original comment, though. The anti-trust case against Microsoft with IE may have gone very differently if it had been about rendering engines on Windows.

This also completely ignores the distinction that it's the App Store which doesn't allow third party rendering engines and not iOS itself (you could create a free developer account and build and run a version of Chrome using Blink if you'd like). So this comes back to the larger question of whether Apple should be allowed to so heavily restrict the installation of software to its own distribution mechanisms.