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.
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