The "CAS" in the OP stands for "Compare and Swap". The complaint is that they had Couchbase checking for read-write conflicts while Mongo was doing blind updates regardless of if the data in Mongo was different from what the client expected.

The complaint isn't about sub-document updates vs. full document updates. The complaint is about conflict checking. Couchbase could have been told to blindly write, but wasn't, and Mongo was told to blindly write.