private subreddits and public discord "servers" both exist.

It’s more about the user intent that causes difference.

I’ve never been in a private subreddit, and the only public Discords I’ve been in are corporately managed with “community managers” and stuff.

Reddit is more of a public forum though, Discord is not.

On Reddit, the upvote and suggestion system amplifies the echo chamber, people are having an entire conversation on one topic. In Discord people usually don't read more than a few existing messages unless they're specifically looking for something, it's far more unorganised and transient.