Even if he said that, it doesn't make sense. It's impossible that a new book outsells Watchmen in just one month, as Marvel is not able to release so many copies in the same month.
Also, in the tweet I read, Cates mentioned that Thor #1 was the most sold comic book of January, and Absolute Carnage was the most sold collection...of January. Maybe he made a joke about being the most sold collection in history, but it doesn't even make sense. How many millions copies has Watchmen sold so far? Absolute Carnage probably sold some thousands