I don't see how writing JSON REST API descriptions isn't practically the same as writing REST API clients anyway: they're still clients, just written declarativ

I don't see how writing JSON REST API descriptions isn't practically the same as writing REST API clients anyway: they're still clients, just written declaratively rather than procedurally. If the point is "stop writing procedural REST API clients and write them declaratively instead" then that advice is by no means restricted to REST API clients. If the point is "hey, I noticed that REST API clients are another thing that we can now comfortably write declaratively" then OK.