It is. The title is a little misleading; here's an explanatory excerpt from the actual paper:

> In this work we analyze the iMessage protocol and identify several weaknesses that an attacker may use to decrypt iMessages and attachments. While these flaws do not render iMessage completely insecure, some flaws reduce the level of security to that of the TLS encryption used to secure communications between end- user devices and Apple’s servers.

TLS is pretty good, but has a completely different scope. It is entirely dependent on the intentions and competence of people who can control the server and access the certificate, as well as the issuer. The client has very little power over this.

A good end-to-end encryption should guarantee that as long as both clients have good intentions, nothing can reveal their communication. This is a much higher level of expectation.