Sounds like a giant waste of ttime, they can share direct deep links in their apps to both contacts
They can .. but The ActivityPub plugin allows subscription to forums so you get notifications in your feed when there is new content on the fourm. Its sort of like an RSS feed. The idea is that people don't have to check each forum they belong to individually.. and perhaps are more likely to see / respond to relevant stuff. - And once its setup you don't actually need to do anything to keep it running.
The "respond" part sounds fishy.
Nothing fishy..Doesn't allow any responses in the forum without people registering as they currently do. Makes a bit more sense if you have some experience with ActivityPub based social media such as Friendica, Mastodon, Lemmy etc. - But basically the way to think about it is a slightly different RSS feed.