Whether or not the software is secure from Apple is a moot point if Apple can, at any point in time, deploy an arbitrary software update that only affects a single device.

I presume Apple has the ability to send a backdoored update to iMessage to any user they want, and probably to obfuscate it well enough to not tip anyone off. Therefore, if DEA can get a warrant requiring Apple to provide technical assistance, Apple has at least one route to get message plaintext.