I don't think Lemmy would count as social media but that said, you can easily create a dummy account that just, for example, posts pictures of moths or beans or bread or whatever.

If you loosely follow the definition of social media to mean “Website where you can interact with people in any way” then yes.

But I think the average colloquial use of the term social media really refers to websites where you make a real life personal profile and share things on that profile. Facebook, Instagram, Twitter, etc. You as the individual are the focus of those types of sites.

The (original at least) point of sites like Lemmy and Reddit is sharing information where you as an individual are not the focus. Hence posting to communities rather than your own wall or profile, the use of usernames over real names etc.

Lemmy also can’t be used non-publicly so a pointless distinction considering the context.

Lemmy is social media, but when people say "social media" they're typically referring to one of two things. They either mean all social media including legacy forums or they mean media that is directly tied to their real names.

I've heard social media where you interact with strangers instead of "friends" referred to as "antisocial media".