Yes that's exactly what I've heard, with such a minute difference in tread depth, it seems comical. By that same logic, everyone with RWD that have tires on the rear that are just slightly more worn than the front would ALL have issues.
I simply don't know much of anything about the mechanics and complexities of the M3, so I would have been scared either way to replace just one tire. I wasn't thinking of wear, I was just thinking if one of the tires blew, or had an unrepairable flat, would I have to match all the other tires.
If you have a flat tire in a model 3, and have 20 inch wheels, Tesla authorizes all wheel replacment tow trucks etc to put a 19 inch tire on there. In fact, I was told all roadside wheel replacement loaner tires from tesla for model 3s are 19 inch wheels, as their engineering stated that a 19inch wheel can be mounted on any model 3 with factory tires on it, no matter whether the car has 3 other 18s, 19s or 20s on it.
I asked for that in writing, and they sent it to me. They (tesla) state there is no issue with motors etc so any issue you would be worried about would be strictly traction related to wear. There is no issue with "the car" having only one wheel replaced, per tesla, specifically in writing to me, when I asked.
Unfortunately, me and tesla roadside are very wheel acquainted, as I have had to replace 4 tires and 3 rims total on this car already due to potholes, where I have never had to do that on other cars on same roads over 20 years of driving.
Anyway, on thread topic, TL ; DR, tesla says its fine, there is no AWD issue to worry about, unlike other cars, but I would still think about wear. No transfer case issues etc on this car.