Thanks, Daphne Maddox!
I think people, including myself, are more concentrated on the idea of emphasizing that we are a “direct relative” from a certain ancestor. And, in so doing, our brains are tricked into thinking that “direct descendant” is synonymous to “direct relative” instead of knowing and distinguishing that you don’t really need to add “direct” to “descendant” because it is simply redundant, especially in our modern day usage. In short, in modern day usage, “direct relative” is simply a synonym for “descendant” without the need of adding “direct” to “descendant”.
“Direct” added to “descendant” just seems (although wrong) to add more emphasis to the fact that someone is a descendant of somebody. I think that Henry Louis Gates is just trying to add more emphasis by using “direct descendant” for the persons involved in his program.