Here’s what happens.
- An activity is received from a remote source.
- We store the Activity, Actor and Object separately in the database (“AP Objects”)
- The AP Objects are associated with their Discourse equivalents, e.g. an Actor gets a User.
In the ActivityPub protocol the real “full Fediverse home address” of an Actor is their id. That is stored in step 2 exactly as its received from the remote source. It doesn’t normally look like @toka@socialhub.activitypub.rocks. That addressing convention, i.e. username@domain.com is derived from the use of Webfinger in the Fediverse, and, while we support and use Webfinger, it is not part of the ActivityPub protocol itself.
In short, we’ll likely be adding support for mentions in the way you’re expecting in the medium term. There will be many issues and edge cases to be figured out in that integration. Discourse mentions are a rich, and Discourse-specific, feature, somewhat different from mentions you’re used to in Mastodon. Nevertheless, I’m confident we can find a way to make them work. Please be patient on that front.