That’s a very reasonable and methodic conclusion. The only thing I don’t like about it is that if you say it outside of the lemmy space it lacks direction. Someone can say “subreddit” anywhere outside of reddit and anyone will either know what a subreddit is, or be able to find out. Talking about a ‘[name] forum’ the listener might wonder “which forum, where is it?”. Even if they know about lemmy.world they might need to ask to be sure that it’s the lemmy forum with that name, that is being referred to. In part because many other forums still exist outside of that space.
What I like about ‘subreddit’ is that it’s still very specific when you use it outside of reddit. On reddit you can easily just say ‘sub’, whereas outside of reddit you’d say ‘subreddit’ to make it very clear, what specific place you’re talking about. If I say ‘[name] community’ outside of lemmy, even someone who uses lemmy wouldn’t necessarily know I’m specifically talking about the lemmy community, unless the context already made that very clear. And that distinction can be helpful in a conversation.