Yes, I realized just after posting this that he was talking about a specific use case (user registration) where the user is the one making the retry in case of failure..
So his example is OK in fact (a variation on the SYN-cookie), I was wrong.
More generally, that process depends on both the database and network, so reliability is already the min(database, network) => network. It makes sense to eliminate a component if you can.
So his example is OK in fact (a variation on the SYN-cookie), I was wrong.