The speed of light is finite, but the same as measured in all reference frames. This has the following consequence. A train is speeding by. Person A is seated in the middle of the train, equidistant from the back and front of it. Person B is on the embankment. When the train arrives at a point where Person A, on the train, and Person B, on the ground, are facing each other, Person B sees lightning flashes striking the front and back of the train simultaneously.
What does Person A, on the train, see?
An object on the train, such as a ball rolling in the direction of the train’s motion, will add the train’s motion to its own motion. It’s called Galilean additivity. But light doesn’t do that. Because it does not add the train’s motion to its own, it follows that the person on the train will see the lightning flash at the front of the train first, and then sometime later the flash at the back.
So Person A and Person B disagree on when the flashes occur. Person A, on the train, thinks the occur sequentially. Person B thinks they occur simultaneously. Who is actually right? They both are. This phenomenon, the relativity of simultaneity, also accounts for time dilation and length contraction.
This is Einstein’s special theory of relativity in a nutshell. It has been tested and verified innumerable different ways over the last 120 years.
This phenomenon is ONLY possible because of delayed-time seeing. If Person A and Person B saw light instantly, they would never disagree on when the lightning flashes struck the train. So we know that what the author claims has no connection to reality.
But this has been explained to you.