"My current gut feeling about all of this is that someone basically was designing a piece of Mac malware that would give them the ability to completely remotely control an infected system. And then they also added some ransomware capability as a way to make extra money."

Honestly I kind of feel like the opposite is probably true. The ransomware is the primary money maker and the backdoor stuff is there just in case there is something interesting to extract if the main money play doesn't net you anything.