They can't distinguish billboards from real threats so the promise of FSD is complete vapor.

Can cameras not detect traffic lights and their signal colors, assisted with mapping data to give a fuzzy idea where traffic light recognition will be required?

Not sure if I would rely on mapping data. There are so many new-ish roads and lights around my apartment. After living here for 6 months one of the roads still isn't known by Google Maps, Apple Maps, or my Tesla's nav software.

If they relied on mapping data to help them know where to look for the lights, I wonder how they'd cope with temporary signals (e.g. at road works)?

You can't drive using maps.