They could rewrite the entire book, keeping only some of the sections with deliberate watermark errors, and it'd still be tracked down to them.
You miss out on the fact that they're not looking for errors - they're looking for specific errors in specific places. Think back to old detective novels with a piece of cardboard with little squares cut in it. Put it over the right page of what looks like a love letter and "we bust out of the back exercize yard at midnight" pops out.
With sufficient redundancy in their data (Come on, people, QR codes, PAR2? ECC? How does a group of computer people not instantly comprehend the idea of redundancy?) you couldn't be sure that random selection of bit flips would be enough to obscure your trail.