That document seems to use communities in the normal English sense and spaces in the “collection of Matrix rooms” sense. I would say spaces are what they’re called and a community is just an informal group of like-minded people who can be better-organized using spaces.
What’s the quality like of the people who are still on Twitter in 2025? Does the fediverse want them? (These are real questions, I have no idea if there’s still any decent people on Twitter.)