> what is this ”new integration architecture” that would make web apps more secure

I think this "new integration architecture" is a very flimsy excuse. To host PWAs, a browser needs the ability to (1) ask iOS for permission to place an icon on the Home Screen, (2) launch certain "tabs" (Home Screen apps) in a separate window. Safari can already do this; they just need let others do the same. Since the PWA is hosted by a specific browser, I suppose you also need a dialog telling the user that XYZ app will also be deleted if they try to delete the browser.

What else am I not thinking of?