I’ve had several BMWs with this feature, and BMW definitely reads the speed limit via the cameras. You can tell because sometimes it gets it wrong, i.e. it will occasionally read the speed limit sign for an alley or some other adjacent road.
Understood. I'm not saying it's technically difficult, it's that MobileEye (now owned by Intel) have a patent that more or less claims the basic concept of reading speed limit signs. I dont want to comment here on the validity of such "do it with a computer" type patent, but it means that anyone doing speed limit sign reading has to either (a) license the patent or (b) risk going to court to fight over it. My guess is BMW chose (a), and Tesla, for the time being, are avoiding either route by using map data and GPS.