OP is from sigmoid.social according to the profile, and that is a mastodon instance. They tooted on Mastodon with the correct @ mention of the community, resulting in the toot showing up as a post in this lemmy community. We can reply to and interact with this specific post, although I’m not sure how it shows up to a mastodon user, seeing as the front end is quite different. We are unable to interact with Mastodon toots that aren’t tagged in a way that tie them to a Lemmy community and create a correlating post