Since I'm a "string nerd" (what some friends call me), I had the same questions. I tried contacting Mr. Harrison years ago...no reply. I tried contacting someone who said he was a dealer. He denied the information was present in any warranty he'd seen and refused to discuss it. Obviously I got nowhere.
There's no reason in the book (that I've seen--I have a copy) for voiding the warranty over a dacron or flemish string.
He gives a few reasons for preferring endless--he says he got 8-10 fps more with endless, that he got more hand shock with flemish, and that a flemish stretched more. My experience is only the latter is true, and then to a low degree if made properly.
He even throws in a dig on flemish strings and those who use them:
"Finally, as a point of trivial history, it became vogue to shoot Flemish-twist bowstrings as a desparate attempt for some over-zealous apologists to return to 'tradition' when compound bows came into being." pg. 414
He then illustrates how he makes an endless string, but that's as far as it goes--no reasons for the warranty being void.
It's been my experience over the years that some well-known bowyers don't understand strings and string materials all that well, and some base their warranty on total misunderstandings. I've tried discussing it with a few of them--didn't do any good.
I can say with full confidence that you won't damage your bow with a (properly made) flemish string. Nothing wrong with preferring and endless string--I have both on different bows, and I make both. I prefer one of the "Fast Flight" materials over dacron, but on my older bows I only use dacron.
If anyone gets more information on why he voids the warranty if you use flemish or dacron, I'd sure be interested in hearing the explanation behind it.
Chad