How does he know this? This is a very weak and high-handed statement to make.
It would seem that all it would take to invalidate his entire argument would be to discover extraterrestrial life.
...or for humans to go into space. But that would be impossible!
Muir_Redwoods, this is called an analogy. An analogy does not prove a proposition, but it can very good for illustrating one. And for illustrating, it does not have to be true; it just has to paint a mental picture that can be understood.
For instance, I could say, "Just like Chicken Little, our friend Cindy Sue seems to be easily frightened by trivial events." You could shoot back, "Chicken Little never existed! That's a fairy tale! So that invalidates what you said about Cindy Sue." --- a pointless objection.
Or another example: I was trying to illustrate for my boys the advantage of careful planning in investing money. I said if you want reliably good bread you have to put in an envelope of yeast that's tested for potency and sold for that purpose. You could let dough set out until it gets bubbly from wild yeast in the air, but that's slow and chancy and might not rise at all.
Son #1: I'll take pancakes.
Son #2: Scrambled eggs for me.
Sheesh. What a homeschooler has to put up with.... :o)