I'm as disappointed in the new rules as anyone, but I'm going to call this one as most likely just a missing implementation rather than malicious.

The new Browser Engine Kit that allows alternative browsers to be installed is really a very deep integration. It's not just swapping out the browser and engine that users open as an app, it's swapping out all system integrations – webviews embedded in apps (in certain types of use), and importantly, PWAs pinned to the home screen. The APIs have a fairly sizeable surface area, and PWAs themselves have many features that require system support to implement.

My assumption would be that full PWA support will return, but that it was a significant amount of extra work that Apple decided to skip for the first implementation. I'm not sure if that was the right call, but they were under quite a deadline with the DMA coming in.