You are right. According to the definition of social media, Lemmy is social media. However, “social media” would by definition fit any kind of digital communication media. A forum, or a blog, or an IRC channel are also, by definition, social media.

I would argue that the social media has a distinct association with Facebook, Instagram and the diverse spawns of those, and by association doesn’t fit anything else. At best, we simply lack a different term, which splits “old-school” stuff like forums and blogs. I view lemmy more like a forum. You have categories, and users can go into categories to start discussions. You don’t follow anyone. People also don’t create and post their own content, but rather seek discussions or share other stuff from the internet. It is social media, but it’s definitely nothing like Facebook.

We simply lack a better term.