It doesn't require it, except that Apple has failed to decouple Safari from iOS, and so it can't be updated like regular apps.

Apple has certain things that they can push any time, such as the lists that safari looks at to find out if websites are dangerous or some of the security stuff they have.

However they ship it, they'd still be in an arms race against google to keep AMP pages hidden from Safari users. That doesn't seem like a productive use of anyone's time.