I find the fact that Web.app behaves exactly like every other application that uses a UIWebView to be very compelling evidence that Web.app is an application uses UIWebView.

The alternative appears to be that to create Web.app they went to all the trouble of reusing the Mobile Safari code, and then methodically went through and broke things such that it behaves exactly like every other application that uses UIWebView.

This would seem like an unreasonable way to get an application that behaves exactly like an application that uses UIWebView.