This is something - well, one of many things that I’ve never understood about the whole flat earth thing, and I’d love an answer, too. How can invisible object block light?
I have to admit that this is a point that I, too, never understood. For all the derision and criticism that the current standard model of science gets from the Flat Earthers, it still has a lot of evidence to back it up. It works, even when the FE side claims it doesn't.
So the FE side must claim that a lot of scientific concept work completely different from the standard model.
Gravity is not based on mass, but on density, or as in the case here, on magnetism. Magnetism itself works differently. Light works differently. Matter works differently.
They cannot explain
how it works differently... it just does, so that their model (whatever it may be) is preserved.
Take two of the more understandable examples from this video: refraction and gravity.
They need to use standard optical explanations to demonstrate the feasibility of their model. The startrails or the sunsets are an example of that. But in doing so, they ignore that these very experiments have nothing to do with reality. They need to place a specifically formed lense between observer and object. But in reality, such a lense does not exist. They just claim "the atmosphere does it."
It is like going to the optometrist, having your glasses prepared exactly for the needs of your eyes... and then leaving without glasses because "the atmosphere refracts light as well."
Similar with gravity. Here, they show experiments with very specific magnetic fields resulting in a certain force. So now they declare: "this form of magnetic field is responsible for gravity." But for other effects, like the movement of the sun, or the explanation of the seasons, they need a different form of magnetic field. For navigation, they need a third variant. And neither of these forms is compatible with the observations... and they can be disproven by very simple experiments.
And in the case of your question: it just does. This object blocks the light where it is needed, and is transparent in all other cases. How? Why? It just does!
In all these cases, they can just claim "It works this way.", and use some scientific sounding terms to give them credibilily. But they never can provide the theoritical or practical framework to show that their version could indeed work.