I believe the reasoning is because light from a lighthouse spreads and part of the spread travels upwards.People can see the light from a lighthouse even with the actual spotlight being obscured.
seriously, for such a big sphere when you look at the curvature of earth from the top of a lighthouse, it's very small, thus the light being able to travel very far
```If those two points were on the round earth it would be a massive distance``` It's a bit of an exaggerated diagram, but the point is even if the lighthouse light is below the earth's curve, its light spreads and should still be visible as a glow on the horizon.
