The single-arm pullover amplifies core activation to the extreme. There's an anti-rotation and rotary stability component that isn't present during the bilateral or isolateral versions of the pullover.
Add in the leg raise hold as I have my NFL athlete Fernando Velasco doing in this video and you'll find this to be one of the most challenging core exercises there is. It's also an incredible movement for exposing and addressing imbalances throughout the kinetic chain.
Mike used to like Functional Training. He used to think Mike Mentzer was kind of a bonehead. Now he's not so sure about the former or the latter, along with a whole lot of other things. If his current rate of "unlearning" continues, he soon won't know anything!