Title says it all. While it is the highest selling single platform game of the year, this does not mean that it sold more than games like RDR2, since it is over 2 platforms. Individually, it did outsell them though. And that is pretty impressive when both combined it's an over 100 million users installbase compared to Switches installbase of about 23 million-ish.
Also in the article: "Further, according to the fine folk over at Amazon, not only is Super Smash Bros. Ultimate the best-selling video game of the year so far, it's the best-selling video game product, which takes into account accessories, hardware, and even digital store cards, the latter of which tends to dominate the Amazon best-seller lists."
However, it is just Amazon as impressive as it is. I am very excited to learn about how much it sold these past 3 days.