I'm highly skeptical about not being able to copy it. Obviously someone like Bill with his reputation has to be taken seriously, so even though I'm skeptical, if he says it can't be copied, I'm very inclined to take his word on it. However, I've done this long enough that it's at the very least confusing. Like Rico said, if it's done on a mill, it can be copied. The only thing I can think of is that if you don't know the technique, you may not know where to measure the pitches from, and that could be the kicker. In a regular drill, all pitches are measured from the grip centerline. In a CLT drilling, when measuring pitches after the line transfer, 1/2 left won't mean the same thing as it does when doing a regular drill. This means that if given a measurement spec sheet, you won't be able to copy it without knowing the technique it was drilled by, but if you have the actual ball and measure everything manually, there's no way in my mind that you would be unable to copy that. Again, coming from a guy of Bill's stature, with his reputation, it would be completely ignorant not to trust or believe him. But depending on your perspective, it could be a little misleading.
Yes, given a special technique and only numbers, I would definitely believe I wouldn't be able to copy it, but given the actual ball, a couple pitch gauges, and a ruler, I'd be really surprised if I couldn't duplicate it. There's also only so many ways to drill something. Again, back to what Rico said, if it can be drilled on a mill (especially with no special equipment required), it can be copied. I do not know the method, but I would have to think that if you're good at fitting people, a comfortable, functional fit should be able to be achieved with no special process or techniques. 1/2 left for a standard drill, or measuring from an unmoved centerline will be different than 1/2 left on a CLT, BUT, it can still be measured based on a standard drill and adjusted for. 1/2 left in the middle finger for a righty using the CLT technique when measured from a standard drill standpoint won't read 1/2 left, it will read something in the neighborhood of 7/16 left and 1/16 forward on a pitch gauge, but given the angle and a little math, you could calculate it out. Meaning that if the CLT for the middle finger was say 10 degrees left of a standard static centerline 1/2 calculates to a decimal number of .500, which is a required calculation for entry into the digital pitch readout on a mill in the first place. If rotated 90 degrees to the left, 1/2 left becomes 1/2 forward, so you have a basis for mathematical calculation. 10 degrees out of 90 equals a percentage of .111 repeating, equaling a rounded percentage of 11.1. If the 1/2 left is then rotated 11.1% left, your resulting numbers are a mildly rounded .045 change per 10 degrees, making 1/2 or .500 left in a 10 degree CLT a converted .455 left, and .045 forward for a standard static centerline drill. Now, .500 left of a 10 degree CLT deviation from standard static centerline isn't going to be exactly .455 left and .045 forward due to rounding, but the difference is in the thousandths, and given that it's hard for people to tell a difference of an entire 1/16 in pitches (.063), there's no way possible to feel the difference.
So in summation, it CAN be copied, though for 100% accuracy, you would have to know the technique. However, an experienced, technically solid driller should be able to get you close enough that you'd have to be a complete princess to feel the difference. I once drilled a ball for a guy whose middle finger and ring finger were fused together all the way up to the base of his fingernails. Obviously not being able to get accurate or definitive fingersizes (it was a conventional drill), and having no basis on which to determine lateral pitches, being that the holes had to overlap enough to keep the grooves at the front and back from cutting him, but far enough apart to still create a grip, I had to completely eyeball it, and I completely nailed it. Now if I can do that, I'd have a really hard time believing that I couldn't copy an already existing fit with all the tools necessary at my disposal. Whether the drilling technique is needed or not, I don't know. It may just be an easier way for other pro shop ops to fit people better, or a more accurate, repeatable standard.