It seems to be a shortcut to open the comment url. Specifically opens the full article but at the height of the comment from which the link was generated, by pressing the icon.
There is a bug and the links are not being generated correctly, because they lack the part of the url that refers to the publication in which the comment has been made.
I will now make a comment to have a practical example.
But it's hard to imagine that it works that way.
Would it, then, be used to republish a comment when the content is accessed from other Fediverse platforms?
Although the link gives an error in the browser, it can actually be used to look for the comment. I used it here from Tusky for Android.
I had to copy the link. I can't imagine what's the context where the icon is used directly without leading to an error, because, anyway, browsing the Lemmy instance, when the Fediverse icon is clicked, a link is opened in a new tab with a 404 error. It's an undesirable behavior.
It could be used to reply, up or downvote, reshare, ... any interaction allowed by that other platform.
Yes, I guess the desirable behaviour would be that clicking would copy the link to clipboard instesd of opening it.
Another idea, so that one does not have to copy paste, may be to open a "remote interaction" pop up that asks you the name of thefedi instance where you want to open the comment in question, similarly to what happens on Peertube