Field-Jefferson-descendant DNA, not necessarily Thomas-Jefferson DNA.
Actually the DNA from the supposed descendant of the Hemings child conceived in Paris -- when Thomas was the only Jefferson in the vicinity -- turned out NOT to have any Jefferson DNA. Also there were other times when Thomas and Sally were at the same place, and could have conceived a child (no birth control back then), but no child was conceived.
Monticello was often closed up when Thomas Jefferson wasn't there. It's only natural to assume that visits from other male Jeffersons would take place when he was there. So it's not surprising that conception dates match those presences (but, interestingly enough, not the one when he and Hemings were too far away for other Jeffersons to visit -- in Paris).
The Goodson DNA failure is interesting, no doubt, because they had the strongest oral history.
Now that we know where Sally slept, and that one would have to go through Jefferson’s room to get to Sally, that negates the theory that his brother was sneaking in from his own plantation 20 something miles away to bed Sally.