That's a pretty narrow view. My Internet in the 90s and early 2000s was on AOL Instant Messenger and MSN Network, Yahoo! Mail and phpBB web forums hosted by random people with pictures hosted on PhotoBucket. Plenty centralized.
The technology doesn't matter, it's changing all the time. The difference is the size of the community, and the barriers to entry. If these communities were easy to discover and join and hard to be booted out from, they wouldn't be protected from The Slop. It used to be just getting on the Internet was the barrier. Now we need new barriers: paywalls and word-of-mouth and moderation.
But AOL IM and MSN were centralized walled gardens that came rather late to the game, and neither PhotoBucket nor phpBB existed at all in the 20th century that is the context here.
The technology doesn't matter, it's changing all the time. The difference is the size of the community, and the barriers to entry. If these communities were easy to discover and join and hard to be booted out from, they wouldn't be protected from The Slop. It used to be just getting on the Internet was the barrier. Now we need new barriers: paywalls and word-of-mouth and moderation.