I don’t buy it. The story has never made any sense in terms of what we know of Jefferson and his nature.
Keeping locks of hair was very common in the old days. I have locks of hair from numerous ancestors. The only one I can be certain of is the one I personally cut of my grandmother’s hair the night she died. Some others could be my father’s, his brother’s (who died in infancy), an aunt’s, or someone even further back.
Does a hair sample provide DNA, or do you need the root?
Then whose hair do you think it is in Jefferson’s book that has DNA that matches with descendants of slaves that are at least know to be related otherwise through mitochondrial DNA from previous work?