This kind of test is mostly for showmanship. There's even a chance that an M5 will get better fuel consumption than a Prius due to the way the test was designed: making the Prius run at the absolute limit in circumstances that cannot be replicated in any reasonably legal setup on a public road, like racing. Even when "cruising" at top speed the Prius should be more efficient than the M5 at the same speed.

An M5 won't have better fuel consumption than a "normal" small car in any situation that you could reasonably see on the road.