EVMan said:
The low powered low resolution camera in the low powered embedded systems with changing roads in extreme light conditions, will be less than 50% reliable ( not suitable for production )
If your i3 had a front-facing camera, you could do what Bjørn Nyland did with his Tesla Model S: cover the front-facing camera with tape so learn how doing so affects the speed limit display. I don't have a link to his video which I saw several years ago, but I believe it confirmed Tesla's description of Speed Assist. Compare speed limit data from map data with that read by the camera. If they agree, display the speed limit. If they don't agree or no speed limit data are available, display the speed limit read by the camera. There might be logic that favors map data if the quality of the camera's reading is insufficient due to factors that you have mentioned. The i3's speed limit system is identical to that used by the early Tesla Model S because they both use the same camera and logic provided by Mobileye.
So the camera is only part of the speed limit detection system. Working in cooperation with map data improves the camera's capabilities considerably. But without the camera, BMW must have decided that speed limit display would be too inaccurate to enable it on i3's without front cameras.