The last step in _configsvrShardCollection is to migrate initial chunks from empty collections to spread them across the shards, which is only done if the collection is empty and the shard key is hashed. If there is a retryable error (like a stepdown) after the step before, which is to actually shard the collection through the ShardingCatalogManager, then when the command is retried, it will early exit successfully because the collection is already sharded with the same options, which skips the moveChunk call.