Pokemon Let's Go is coming out in two months and from what I know it doesn't have quite the hype that previous Pokemon games have had, which means that this was probably meant as a marketing trick. The idea was to reveal a Pokemon in order to start a discussion, which it did. Everyone was talking about this mysterious monster on social media. More people hearing about the games leads to more people getting interested in them and potentially purchasing them. It was clever because it was mainly aimed at casual Pokemon Go players that generally don't play the main series games. Some of them might not even have heard about Pokemon Let's Go and this was a way of announcing it to them.
Since Pokemon Go was such a success, I believe that Pokemon Let's Go is nintendo's attempt to bring as many Pokemon Go players as possible into the main franchise. Announcing a new Pokemon through Pokemon Go would make sense.