Notably, these web apps will "continue to be built directly on WebKit and its security architecture", so a progressive app added to the Home Screen can't for example run on a Chrome engine.
Yeah, that is interesting to me. I would imagine one of the main reasons people have been asking for alternative browser engine support in the first place is the potential to have more features added to PWAs by competing platforms, since they were clearly not a priority in WebKit for a very long time.