1) the detailed definition of Parallel is that if you drew a line from the crease of your hip to the top of your kneecap it would be parallel to the ground - not really the same as "thigh" parallel to ground.
2) I think if you can go below parallel without losing form you should feel free to do so and not stop at parallel, but the purpose of writing that was to make sure you go low enough rather than stop you from going too low (if you see what I mean).
3) if you look at the cover art from the book Starting Strength, you can see that the setup position for the DL and the bottom position for a squat look different. This is why I think it is important to do DL using the bar (and not trying to do it with dumbbells).
eta: jinx missjane