Reading it I imagine it's roughly because they started with the problem of "we have to async writes to postgres for scale" and then solved it with "we synchronously write checkpoints with enough performance and guarantees to postgres to solve scale". The middle bit was likely quite hard.

Agreed it's missing that detail. I think it makes sense though that the durable queues shouldn't need strong consistency and transaction isolation, just durability, so the DBs can probably be sharded pretty arbitrarily, maybe operate in lower isolation modes, etc, whereas the DB they need to async writes to probably does need transaction isolation and all that. I'd appreciate if the article would confirm or deny my guess here!