As part of the kCheckShardPreconditions phase in AddShardCoordinator we set the fcv of the new replicaset the same as the cluster. If this command fails, then it's clearly an incompatibility.
It would be nice though to put the fcv setting into the kPrepareNewShard and keep the kCheckShardPreconditions as purely checks (no modifications made).
For this we have to find out a way to get the latest supported FCV version from the replicaset and check if that is equals or greater than the current FCV on the cluster.