Nobody on a Lemmy instance is able to follow accounts (like on Mastodon or /kbin). Thus, Lemmy will not fetch anything from Mastodon unless written specifically to a threadiverse community (and the community being CCed). Because of this, Lemmy instance are less harmful, than (potentially) any microblogging server (be it Mastodon, Pleroma, Soapbox, *key, etc.)