Because I do not consider signature wins to only come from perennial powers. It is about the here and now.
This is what drives me nuts. People cant have it both ways. For example, many so called experts were picking Nevada to upset Notre Dame, but when the Irish won convincingly, they all said, well its just Nevada. Either you give the Irish credit for beating a team that many thought would win the game outright, or just say Nevada sucks from the get go. You cant have it both ways.
So lets transition that argument, to this one. In 2005, Notre Dame went on the road and beat then #3 Michigan. But no one considers that Weis's signature win because Michigan went on to finish 7-5 that year. So that tells me a signature win has to occur against a highly ranked team. Pitt should be between 10-15, and it is on the road. Unless I am mistaken, if we win, then this will be the highest ranked team that Weis has defeated on the road since that 2005 game against Michigan. So again you cant have it both ways. If a perennial power struggles, you say its not a signature win. But when you beat a highly ranked team, well they arent a perennial power.
So to me, when considering all of that, it is a signature win. You are correct in saying that it would not be a signature win for Notre Dame. Maybe that is where our arguments differ. But at this point in his career, it would be Weis's best road win, and thus the signature win of his tenure to this point at Notre Dame.