The blog post dives into how it's hard for spammers to automate adding themselves onto the whitelist because its a chain of trust. You have to have an existing instance owner to vouch for you, which they can revoke at any time. A spammer couldn't do things like run a "clean" instance, and then whitelist off that, because presumably someone would try to contact the owner of the presumed "clean" instance to get them to remove the spam. When they don't respond, or only partially address the issue, it's possible to pull rank and contact the person further up the chain of trust.

In short, it's real people talking to each other about spam issues, but in a way that scales so that an owner of one instance doesn't need to personally trust and know every other instance owner. It should allow for small single user instances to get set up about as easily as any other instance. Everyone has to know and talk to someone along the chain.

The real downside of the system is that people are human, and cliques are going to form that may defederate swathes of the fediverse from each other. I kinda think that's going to happen anyways though.

A chain of trust is the best proposal I've seen for addressing the scaling issues associated with the fediverse. I'm not associated with that guy at all, just saying I like his idea.

-- edit

On second thought, getting your instance added to the chain of trust is literally no more difficult than signing up for an instance with a questionnaire. It's basically that but at the instance level instead of the user level.