It's not about browsers. It's about OS APIs that browsers need to support PWAs. Apple is killing those APIs so that other browsers cannot offer PWAs.

That's what I meant: they could continue offering safari as a "basic web browser" but allow actual web browsers in the appstore, that can fully implement any web APIs.

This is probably not accurate given I was mistaken above but I thought that's what Apple was doing. You can load any browser but some OS APIs are going away.