I don't know why they think INTP = sad eyes, but at least I'm interested in investigating the issue. It's as if you're saying, "I don't understand why people say the Shroud of Turin is 2000 years old." Maybe someone should find out for sure? And so they did. Why the instant skepticism? Why not at least give it a chance? Skepticism works both ways.
But I haven't investigated the sad-eyes issue very much in this thread, I got side-tracked by long vs. round faces which is also very interesting. Yesterday I walked through a grocery story looking for round and long faces (I've never done this before). It was rather busy and 99% of the people in the store were women. I asked my wife (ISFP), who was with me and has zero interest in typology, if I have a round or long face. She said "long, but not like Abraham Lincoln's." Most of the women in the store had more round faces, including my wife.
To me this is an interesting topic as a clue to personality typing at a glance.