(SPOILERS) SuperDEATH Kills the competition

[Mardoc]

But a lighthouse gets you, at maximum, one food per population point.

Ah, that's a good way to think about it. The two should be equivalent in a fishing village with literally no food resources - then the lighthouse and the granary both cut the turns-to-grow in half. If you have a resource, though, then the granary is doing a lot more since it doubles the food still, while the lighthouse only adds its usual one per tile.