No you don't get me but all browsers in Iphone even firefox and chrome are webkit forks
and neither of them allow any sort of extensions on top
Orion is the only one I think which still supports firefox or chrome extensions as well. I am sure that it can support PWA or already does, not sure, someone should probably test it out.
Theoretically if you can modify the engine enough to run firefox/chrome extensions on it when firefox/chrome themselves on Iphone can't but somehow Orion can, I don't see a reason why nobody's else doing it but combined with some really really good pwa support as well?
I don't think you get how it works. When you download a browser on iOS it does not have an engine _at all_, not even a "WebKit fork". The browser is just a UI and wrapper for one of the engines bundles in iOS. No modifications can be done to the engine whatsoever, it is part of the OS.