This place has roughly 3,000 people and was intended to be an entire replacement for DaystromInstitute and StarTrek as they were going dark indefinitely. Well, within 4 days the moderators have walked back those statements and opened both subreddits up. I see no incentive for people to come to this website now and while a few may come here in the future, most people will go to r/startrek with 600,000 people.
Things make a lot more sense once you realize that “instance” is just a hand-wavy synonym for “website”.
“That community is on another website”.
“When I click a link, it takes me to another website”
Etc.
Watch the url. If it’s still showing you’re on the website you have an account on, then you’re working with locally mirrored content, and can comment without barriers. If it’s the url of some other website, then… It’s some other website,and you’re probably not logged in there unless you have an account on that site, just like following links to Twitter or Facebook from Reddit.
The only real differences here compared to centralized social media is that other websites will share the content directly with each other. If they’re asked to.