This sends the file in transit over SSL, but encrypts the data at rest on EC2. That means you must completely trust EC2.
Worse, based on the comments in the blog, it is unclear who has the key that is used to encrypt the files: hotzyco: "What key AES uses to encrypt files?" Response: "Sorry hotzyco. We can not give out that information."
If Zumodrive controls that key, they have access to your data.