The forums is configured to be “Web App Capable.” Basically all this does is takes the forums website and puts it in an App window and gives it it’s own App icon and title. So basically you’re just looking at a stripped down version of Safari that only shows the forums site. It will still apply rules from Content Blockers. But yes, otherwise that just puts a link that sends you directly to Safari.
Incase anyone is interested, Apple has some legacy documentation on Configuring Web Apps (you may have to to minimize the table of contents list when you visit the site).
I’m fairly sure it has to be storing data somewhere. The normal forums website does:
Also, you are able to clear the cache of web apps when you press the “Clear History and Website Data” button in Safari’s settings page. However, despite there being web apps having somewhat of a connection to Safari, when you use just the web app version of the forums, it doesn’t appear to show any data being stored in Safari’s Website Data page in Settings. But even after you clear Safari’s history and website data, it seems to keep you logged into the forums site.
So in summary, it has to be storing data somewhere. Otherwise it couldn’t keep you logged in and it would forget the app icon and title. But I don’t know where it is technically storing it or how you find out how much data it is storing. Either way, I highly doubt it’s a very considerable amount of space being used.