This is a lie. PWA in iOS basically limited to a link in the Home Screen opening a web page on the Internet without Safari controls.
So in practice one can’t install web apps in iOS. Apple tech to support PWA installs is really feature poor not to mention non standard. Safari approach to the support of PWAs is like IE support was more than a decade ago regarding the web when MS aim was to funnel the web tech into into the Windows OS ... it was stopped from being able to do so by regulators. By assuring that in Windows people could install alternative Browser / Engines by default leading to more innovation than IE could keep up. IE, a cancer that even today has impact in LOB apps.
For instance in iOS, PWAs cannot rise notifications like apps do, local cache is very limited, opening a we page in Safari does not direct to the PWA installed and many other things that other browsers/OSs support. Default apps ... None to do with security. The reason I guess its because if they did supported as it could and should be done, as SJ initially thought it should be done, many many apps would be out of the App Store.
The difference between Safari PWA support and Native apps on iOS is the same has between paying to watch a movie in a big screen or trough a tightly controlled key hole. Yes, you have options, but in highly competitive market like digital services, servicing customers trough a key hole is not much of a viable option.