Mongostat 2.4 supports --discover on both 2.2 and 2.4 clusters. Using a 2.2 version of mongostat with --discover on a 2.4 cluster it will only see the mongos and not mark it as a RTR.
We should probably either make 2.4 mongos work with a 2.2 mongostat or backport the change that makes mongostat work to 2.2.