It's just one of his moronically-recycled word games, where he equivocates the meaning of "proof." He'll use it as though it meant "demonstration of metaphysical certitude," but when challenged, he'll claim he only meant "evidence." He's used this bait'n'switch tactic dating back two years, and was called on it back then.
Yet he's back yet again to make a raving fool of himself, using the same tired, fallacious argument......
But what can one expect from a person who thinks that nuclear fission is a "chemical reaction," that "a circle is not an ellipse," that "1720" is a really big number, that the planets whiz around in "wildly elliptical" orbits, and that infrared radiation causes sunburn?
Don't forget that "there are no plant phyla."