MongoDB WiredTiger RAID EBS back ups

The MongoDB documentation says that for nodes backed by EBS volumes in a RAID array, the back up options are either:

1. Lock writes to disk via db.fsyncLock(), take an EBS snapshot, and db.fsyncUnlock()
2. Use LVM

However, it says that option (1) is only supported for the MMAPv1 storage engine. Why is that option not supported for other storage engines -- specifically WiredTiger?