The author considers speed of light being the absolute, irrevocable certainty, but how does he know that?
Does he know _why_ c is constant? Had he mastered general relativity enough to be 100% sure that it can't vary in time or space? Or that our current understanding of physics is total and there absolutely can't be something that we don't know about speed of light?
They know it because it's the best theory we currently have. Things aren't 100% sure in physics, there are no laws. Just the best available theory which you should treat as fact while knowing it may not be.