Even though I don't understand any of the science very well, I know enough not to attribute these mysteries to magic or gods.
A chimeral person with two different DNAs in their body isn't magical, but it can throw off any rule base system that says "People only have one DNA in their bodies". The first chimeral person that showed up was a type of black swan event showing us that we did not understand the problem space as well as we thought.
> how in good conscience could he act on basically "taking the word of some guy in a white lab coat."
is actually quite correct. There is science behind DNA evidence, but there is also frequent malpractice of lab technicians, which ends up muddling results, with life ruining consequences.
John Oliver has talked about it: https://www.youtube.com/watch?v=ScmJvmzDcG0