Their test was to loop non-stop around a race track at the Prius' top speed, which is pretty much what the sports car was designed to do.
They neglected to do a round in stop-and-go traffic, which is what the Prius was designed for.
Edit: it probably didn't come across like it, but I think that the show is hilarious and it is one of the very few reasons to own a TV. But a car review show (as in, the source of information on the automotive industry) it is not, and is not meant to be.
Sure, the Passat is a bit of a slug, but light-footed, I made an estimated 50 mpg in Italy, 4 up with luggage, AC on, during an Italy trip, something you couldn't do in a Prius.
They drove the Prius as fast as it would go, because someone who buys a Prius will do that and at that speed it would be less efficient than an M3 which is the "sports car" you mention.
And I agree with that point. If I flog my Civic, it'll return much smaller mileage than if I drive carefully and efficiently. So, again, Top Gear's point is proven in that, people aren't willing to think critically and live in a reality.
I don't think anyone would dispute that a Prius is more fuel efficient than a sports car in stop-and-go traffic and having that segment on Top Gear just wouldn't be possible as it would make the show "fair" to the hybrid and actually about cars which it definitely isn't.
And I think they make these silly claims at times since they know they are so over-the-top that it's hard for anyone to take them too seriously.