Evaluating primary sources isn't a perfectly simple and straightforward exercise, and cranks generally have facts that they can point to to "prove" that their view. I expect that with selective enough citation I could create a fairly convincing (to a layman) argument for quite a wide variety of points of view.
Comparing primary sources is indeed not straightforward, but this is primary vs. secondary. If a book makes a claim that is directly contradicted by the author's own source then not much expertise is required to see that preferring the "child" over the "parent" would be absurd.
