The story doesn't mention that the logo came from a forum on a site called Armchair General. The logo was the start of a joke thread that announced the image as the new Democrat Party logo. Chances are that Google saw this site as being somewhat reputable/popular, found the image, used the phrasing as a signal, and noticed the post time. Those could have been enough points to automatically get the logo updated. At least that's my assumption so far, anyways.