I do not use iMessage, so I should be ok, correct?
This one is not about the iMessage vulnerability but that was not really a problem at all. To gain the iMessage key required someone with actual access to an iPhone to send the same photo thousands of times through a man-in-the-middle fake server to another Apple device, subtly changing the photo each time so they could see what the server got each time. From that, they could eventually figure out what the 128bit AES encryption key was. This was not a likely scenario for anyone to intercept the average user's iMessages.
If you sent any messages at all on an iPhone, you've used iMessage. It will either go normal message through the cell carrier, or encrypted iMessage through Apple's servers.