Subs with smaller woofers can be very impressive. With better design, they can put out some solid bass. And they are usually tighter, because of the smaller woofer.
Where they usually fall short, though, is in the deep bass. I haven't heard the sub linked above, but I have heard more than one brand of smaller subs, including several versions of the Definitive Supercube. I have yet to hear one that clearly reproduces the really low end like a 12 or 15 inch will do.
So though I haven't heard that particular sub, I am skeptical that it is the best home theater sub on the market. I suspect it is actually a better musical sub, and less of a home theater sub, because it probably struggles with frequencies below 40hz.