Yep, but we need the genomes of two critters that have descended one from the other. And we don't have the standard to tell entirely what data is correct and what data is in error.If we're comparing the DNA of two critters, each one of those sets of DNA is what we're trying to read - each is the "gold standard" in my example. In an ideal world, we can read each perfectly. In the real world, we can make the error rate as low as desired, as Tom Schneider said in the very next sentence after the snippet you quoted.