Actually, depending on the length of the Pernese year vs. the Terran one, not so long as that. The colony ship may have taken as much as a couple of centuries to get the colonists in hibernation to Pern (although I think it was more like a couple of years), & the Pernese year/Turn undoubtedly differs in lenght, but is it longer? 1500 +/- Turns could be no more than 900 Terran years, although it's probably fairly congruent. The drift wasn't bad when that follow up vessel showed up about a generation, generation and a half after their first Threadfall.
But, the science should give Todd room to play. I just think he needs to bring in a co-author who's more in tune with story flow if he's going to do that.