I disagree. The challenge, as I read it, is to produce evidence of paranormal or supernatural ability or phenomena. If you claim to be able to make a truck levitate but only manage to make a VW bug levitate would that satisfy the challenge? Not with your logic. Would it still be evidence of such phenomena? I'd say so.
My point is that requiring more than is scientifically necessary undermines the credibility of the organization and gives claimants an arguement they shouldn't be given.
We've had this discussion on the forum before. If someone comes in claiming to predict coinflips with 100% accuracy, then Randi can *and will* test them by asking them to predict just 20 flips. There's a 1-in-a-million chance that a non-psychic person will win by dumb luck, by making a lucky 50-50 guess 20 times in a row.
Suppose that this person takes the test, predicts 20 flips, and gets 12 right. Is this, as you might think, "evidence" that the person really does have psychic powers with 60% accuracy instead of 100%? No, not at all: due to the laws of statistics, a random-guessing person will get 12 right sometimes: you probably have a one-in-five chance of passing this "test" with no powers at all.
60% accuracy is testable ... if you design the test to look for 60% accuracy! If you walked up to JREF and asked to be tested on 60%-accurate coin-flip-prediction, they'll ask you to get 750 right answers right out of 1250 trials. (A random guesser will get 625 right on average, with only a 1-in-a-million chance of getting lucky up to 750.) If you only get 650 right on this test, does that mean that you have 52%-accurate ESP? No, it means that you got lucky at the one-in-three level; it would take thousands of trials to convince anyone that you actually had 52% ESP.
Designing the test to test the claimed ability is absolutely scientifically necessary. That's how science works---even when you're searching for unknowns, you have to make concrete decisions about what equipment to use, how long to search, how much noise to tolerate, etc., which are informed by a) what sort of things you hope to find and b) how much money/time/energy you have.
The point about the VW-versus-truck is a bit hypothetical. I dare say that if someone came in, tried and failed to levitate a truck, but successfully levitated a Volkswagen, a person, or a rock, it would suggest two questions. First, why the heck didn't they volunteer to do a rock to begin with? Second, this sort of pathway could be a magician-like bit of misdirection---for example, while Randi is inspecting the truck for trickery, your confederate could be attaching a hidden monofilament to the rock, or attaching some sort of sneaky Balducci-style levitation device to the VW. You would never award someone $1,000,000 for doing something amazing and unexplainable off to the side, while everyone's attention was elsewhere---that's what stage magicians do every day.
If it happened, though, I dare say they'd be invited to reapply for a rock-based levitation test---one which gives Randi time to inspect the rock---which they would pass if they actually had psychic powers.