One thing to remember is you have at least two other ways of storing values. Each worker has global memory which persists between invocations (local, atomic), and you have access to the Cache API (per-data center, ephemeral).
By combining those with KV to do global coordination on a more course time scale, I think you could.
Rate limiting doesn't have to be coordinated to work. But you do need a PN-Counter, which they are only considering at this point.
A PN-Counter is a method of coordination. And if the writes were coordinated with the consistency time, that would be sufficient to implement consistent counters (or even a naive counter in a store given atomic operatons).
PN-Counter is coordination-free counter, it can't be used as a method of coordination. But it's ok for global rate limiting, as it doesn't have to guarantee exact rate at any point in time, as long as it eventually averages on specified rate.
One great example of why this would matter to me - could workers ever institute a coordinated rate limiting function using this KV store?