Although there had been a couple unofficial contacts between Humans and Aliens previously, they were not well documented at the time, and didn’t become common knowledge until long after the contact with the Tractus Canis, (or, if you’re pedantic, Intercapedo Canis) so the Canis indecent remained fixed in everyone’s mind as the ‘first contact’, much as Lindberg’s crossing of the Atlantic was remembered as a first, or Columbus’ discovery of America was regarded as a first: none of them actually were, but 99% of the world’s population would swear they really were.