Storage engine on a node using ordered keys is not the same thing as a distributed KV store maintaining an ordered index.
In general, a single node partition will use ordered storage. At a higher level (partitions/shards), you use either key ranges or hashing to route keys to partitions: https://tikv.org/deep-dive/scalability/data-sharding/
It seems TiKV uses range partitioning, so that's giving you the order across nodes.