Um. Nixon was never impeached. The only president in the 20th century to be impeached was Clinton.
While strictly true, functionally Nixon is the only president to ever be removed because of impeachment proceedings. He knew what was coming and stepped down because of it. I don't see how stepping down because he KNEW that an impeachment was not only coming but that the conviction was as well somehow makes the fact it was going to happen go away.
It is a true statement to say only Clinton was impeached in the 20th century. It is an untrue statement to say Nixon was impeached.