Actually I am almost certain that is a modification of the Cata installer.
Either way it is a very poor attempt at a fake. The very first thing I noticed was the inconsistent font for the "You" at the bottom. Aside from being almost the correct color, it doesn't match up to the rest of the sentence. And the spacing in relation to the rest of the sentence is wrong. I also do not think you would get all of those elements of the installer on the same screen like they are.
Also, from a brand new account named Comingsoon, who only has one post. If that doesn't scream trollbait, then I don't know what does.
Oh yeah, and the install size. MoP installs at over 20GB. They are not going to magically shrink the size of the game install by half. Previous betas were always the same size or larger as the current live version.
Very fake guys.