I think it has to do with activitypub, where Mbin follows the link, extracts some activitypub details from it, and then displays a link to the local version of the thread based on these details rather than showing the link itself. They also extract community icons and the user name of the OP.
It seems it might not work consistently 100% of the time, but when I've seen it it has been pretty useful, and it should at least be possible in theory. :)