I signed up for a couple of Fediverse instances that interested me; Lemmy, Mastodon, and Mbin. The app I use on my phone for Mbin is called “Intersteller”. When I filter by “all” posts on Intersteller, it shows Lemmy posts as well. Are they the same instance? If not, why is it showing Lemmy posts?
You seem to be confused what the word “instance” means in a fediverse context. The answer to your question, as asked, is “no, the question doesn’t even make sense”.
Lemmy and Mbin are two different backends that talk in the same language (“protocol”), ActivityPub, to each other. The entire idea of that common language is that you should be able to use either of them and also communicate with people using the other.