I think of ActivityPub and ATProto as optimized for different scales and full-network searchability. For something small to medium (up to hundreds of users, maybe low thousands), ActivityPub is in general a lot cheaper than an ATProto AppView (which is what gives you the full-network searchability) -- especially if you're using something like GoToSocial or snac. There's a bit of an apples to oranges comparison, though, because ActivityPub doesn't get you full-network searchability.
[If you're willing to give up on full-network searchability -- or rely on an external service like the microscosm -- you can have a partial appview, which makes the cost structure more ActivityPub-like, and I see a lot of that happening with formats other than micro-blogging. But that's not what Blacksky's trying to do.
On the other hand once you get up to tens of thousands of users, ActivityPub gets expensive too. And if you're actually treating moderation time as a cost (as opposed to viewing it as unpaid labor, which most AcitivityPub instances do), it's likely to outweigh the storage/compute/networking costs. Blacksky's invested a lot in moderation tech and processes, which gives them a big advantage over most larger fedi instances., Then again most larger fedi instances don't moderate well so it's another apples-to-oranges comparison.
@fromjason @ohmu