Jack Harrisson would void the warranty on his bows if you used anything other than an endless FF string. There was also a thread here, not too long ago, claiming flemish strings caused limb twist. It was locked, but I'm sure you can find it.
So, is it possible the guy read that about dacron strings? Yes, it is very possible--it's in Jack Harrison's warranty (or at least was, in the copy I read), and I think it's in his book as well. Goes to show you can't believe everything you read.
Is it true that a dacron string can damage a bow made for FF strings, just because it's a dacron string? Not at all. That's as ridiculous as blaming a flemish string for a twisted limb. In my opinion, of course.