It doesn’t seem like that to me, as the Fairphone link looks and behaves in the same way as the Discourse link.
How can I check for a difference here?
This is not the result of really adding a link to the home screen, which would not change the URL in question to the respective index page of the website, and which would not result in the link then being opened without the browser UI.
I have “real” deep links on my home screen which get opened in the browser with its full UI and I know it sometimes wasn’t easy to get them 
… Firefox 82: Force page shortcut onto home screen instead of installing a PWA
I read that, but the question is what this support actually was.
It sounds to me like apart from the presentation of a website as a PWA, which Bromite and Firefox still do, Chrome would do more.
My question is whether that includes something substantial worth having. I can perfectly live without PWA entries in the App list, for instance.