I believe the author was a little naive in thinking that the lens would not have to be stopped down for astronomical purposes. Stars are the ultimate challenge for any lens. In fact, every lens has to be stopped down between 1 and 2 stops to get good stars in the corners. As a result, the 200mm f/2.0 still has a speed advantage after stopping down, and the four inches of light gathering ability is nice too.
I concur. The lens excelled for his (way too many) cat, dog and baby photos. Short of a Schmidt camera, nothing is going to do well for astro at f/2, and it requires a curved imaging surface. The old 300/4L (non-IS), discontinued now but arguably Canon's best affordable long lens, has always been considered sharp, but even it is best used at f/4.5 for astro. I'd love to have a 200 that I can only use at a measly f/2.8 for astrophotos!