Earlier quoted context omitted.
Surely the uniqueness is only required at the bottom end of the stack before the first 'router' ie the cell tower
Yes, but you could have hundreds of thousands of devices connected to that tower in a dense metro area. Even a single hardware address collision would result in both devices being unable to reliably reach emergency services.
It seems a number of devices are now randomizing their MACs when connected to wifi. I'm not sure how they handle address collisions, if they do at all. Most wifi APs serve a lot fewer devices than a cell tower, but it's possible that the issue isn't as significant as I thought.