> it fails to answer "so how does atproto solve the problems that defederation solves?".

The better way to ask this is, how does ActivityPub solve the problems that defederation causes? It's essentially the thing Microsoft does with email. Discard messages from all but the largest providers, defederate by default, forcing users to use Microsoft or another major incumbent if they want their messages to be delivered. Then new instances can't have their messages delivered, therefore can't get users. Which is obviously a perverse incentive for the major incumbents to not federate with new instances.

It's an architectural choice that has the long-term effect of cementing an oligopoly.

Meanwhile the claim is that it's necessary to prevent spam, but there are other providers that don't do this, e.g. in general you can deliver to Gmail as long as you have DKIM and reverse DNS etc. configured correctly, and those providers don't have any more of a spam problem than the ones who block innocent small servers by default.

Moreover, there is an obvious way to do this without giving the major instances a perverse incentive. You do the filtering on the client so that the filter list(s) you use are provided by something in the nature of uBlock rather than something in the nature of Microsoft, since the former doesn't operate any instances and therefore isn't trying to pressure everyone to use theirs.