Then take the spirit of the recommendation and not the letter, why allow anything after the tld?
Now, yes, a site could spin up unique subdomains, but, that's going to be harder work and would probably put most off.
Could not even allow sub-domains, you want a PWA, it'll have to run from the main domain.
That said, sounds a lot like a non-issue, if you like a site enough to use a PWA and add a shortcut to your homescreen you're probably going to want to be logged in to it anyway (like a weather app so you have your location stored, or twitter so you're logged in etc..).
Biting the hand that feeds IT
