I think this is meant to be the explaining sentence: "We now think that many genetic changes were happening that conventional DNA analysis did not capture."
To me this means that the longer time period includes heretofore undetected changes.
1) that DNA change is faster than expected.
2) that the amount of DNA change that you might expect from 20,000 years divergence is actually 40,000 years worth of divergence (i.e. the DNA change is slower than expected).
And in neither case is the evolution fast enough to satisfy the needs of Creationists who, although they actually believe in evolution thousands of times faster than evolutionary biology suggest is possible - usually say they don't believe in evolution at all.