I am much less knowledgeable about DNA tests than many of the other people on this group but I think less than 1% could likely just be a statistical artifact and might not be real. I assume the record that you found for Hans Koch was in a Jewish database or you have some other reason to think he was Jewish, in general the Ashkenazi custom is not to name a child after a living person so for the father and son to both be named Hans (if they also shared whatever Hebrew/Yiddish name it corresponded to) would be unusual but not impossible. Without seeing the record it would be pretty hard to figure out what exactly it meant. Also as a side note if the marriage was in 1674 it's unlikely that he was born around 1600 because he would be quite old especially given the life expectancy of the time, but anything is possible. Being able to see any of the records would help to try to clarify more.
It's really amazing that you have family trees going back that far, I wish my family had such an extensive tree
SPIELER Lodz, Zloczew, Belchatow
SEGALL, SCHWARTZ Piatra Neamt