If the dude was able to come up with a QR-code for an extortion letter, he/she is most likely IT literate
Not necessarily. QR codes are pretty well known. I mean the things are everywhere and a quick google search will tell an interested party how to make one without them even having to understand the underlying concept. But someone with no knowledge of networking technologies would be unlikely to have come across the term "MAC address".
Mind you, if they didn't know what a MAC address was before then they do now and are no doubt either spoofing a new one - an easy task to figure out once you know that it's possible - or shopping for a new phone.