These instructions don’t appear to be documented. Here’s a post from a bit while ago that covers the basic functions of participating on Lemmy via Mastodon mentions.
It’s true this is a thing that you can do, but the experience seems pretty degraded vs. just registering an account with a Lemmy/Mbin/PieFed/Sublinks (did I miss any?) instance which is natively configured for the kind of threaded conversations that exist on this segment of the fediverse. The instructions basically amount to “Go to a Lemmy instance and use its interface to find a community you’re interested in, then copy the link to the discussion you want to interact with and paste that into your Mastodon instance’s search bar, then reply to the post that appears. It’s that simple!”
If you only interact with threads occasionally or you just want to try it out from Mastodon, this is workable, but you need a lot of patience for the busywork that’s involved.
Lemmy & Mastodon are completely different beasts with different formatting and function. This is a terrible abuse of ActivityPub IMO and degrades both services
Imo, AP was created with a great deal of integration in mind without being prescriptive. It’s hard to gauge what abuse really is when the feature in question is marginal and a bit inconvenient to use as other commenters have pointed out. I look at it more as a “look what we can do” type of thing.
