Was this taking a MongoDB database app and pointing to CosmosDB or was this developing a app around CosmosDB's partitioning etc but using the MongoDB API for read/write?

The former, lift and shift scenario I can understand as you need to structure your data to work best with CosmosDB/DynamoDB etc. If it's the latter that is useful to know as I'm currently building a POC on Azure so I should watch out for it.