He clearly said this because he was anti-Nixon. Obviously, the word "launder" has always been connected to things that are dirty. After all, there is no need to launder something if it is already clean.
So, I don't really see the irony here. It is somewhat ironic that a person who ought to have represented law and order (by his own claims) would end up giving us a new term for a kind of crime. However, I don't really see that using the word "launder" is ironic because it has always been used to taking something that is dirty and making it clean.