Oh right. No, it doesn’t work that way in my experience. I’ve seen Mastodon users post to Lemmy Communities by @ mentioning them, and Mastodon users replying to posts. It often looks weird because replies have an @ mention (or multiple, in the case of replies to replies), but everything shows up in Lemmy how you’d want it to.

Why yikes? The scenario you described is literally the same as 50,000 users signing up to Lemmy to post in a Lemmy Community.

IMO bridgy is not well designed. The fact that it requires both the follower and the followee to specifically opt in basically makes it DOA. Both Mastodon and BlueSky are completely open and public in terms of post visibility, so bridgy should have been designed to require explicit opt outs from anyone who didn’t want their content bridged.