They said that was their time budget, but didn't follow up with how expensive it ended up being in practice -- but agreed.

The article says they were able to keep it close to their max budget, which actually suggests that it was higher than 10ms in practice

And presumably query the replicas until an up-to-date one is found (based off the LSN of the user).

I’d have expected the LSN of the replicas to be in redis as well, but fair point. However even as a worst case given their number of replicas, I’d expect to be able to hit redis AND each replica in under 10ms