Well, I suppose literally he's right, but then again we don't know the precise composition of the earth either. We often times can infer the composition of bodies in space by examining the spectrum of light coming from those bodies just like we can infer a lot about the nature of the earth's crust and core but the patterns of seismic waves generated by earth quakes. I'm no expert on the Grand Canyon, but I don't see any evidence of volcanic rock at the canyon. Scientists infer instead that the Grand Canyon's formation is in large part due to erosive forces operating over tens of thousands of years. Scientists do this by extrapolating laws that govern the current universe back through time.
Astronomers do something similar when they study the stars.
Remember, under current theory the light from stars has traveled sometimes many thousands of years. If you want to get closer to home, even light from our sun takes 8 minutes to reach the Earth. I suppose Mr. Willis might argue that you can't prove that the laws of physics apply to the stars, or prove that the speed of light is constant. He would be right. Maybe the Sun has just been wiped out.