What often happens is that the conduit carrying the power to the light is improperly installed or crimped after the light is in place. If that's the case, the only fix once the power cable breaks is in fact to bust concrete. The work-around I've used is a wet-splice of the power cable, but that's technically beyond most pool guys. The PROPER way to do it is install the conduit correctly, but that's rare.

However, I don't know if that's what happened.

You should be able to check, though. If the wire for the power supply still is in place on the box end of the conduit, but is missing from the niche, the cable probably broke when they were trying to extract it. Possibly, someone more skilled could have gotten the cable out, without breaking it. But if the conduit is broken or crimped under the deck, that's unlikely.

I'll spare you my rant, but if pool owners would spend 10% less on 'cool stuff' and 10% more on invisible construction quality improvements . . . this sort of crap would be much less common. But "cool" is easy to sell and boring "quality" (better valves, pipes, methods etc) is not.