They used Yoga Socks. These aren't your average slippery cotton socks, they are designed to provide a secure grip for your feet. In actual performance I have no idea how they compare to a sweaty foot for slippage (oh yes, the previous posters failed to mention that potential negative of barefoot running grip...).
I'm surprised at all the zealotry going on between the camps. Barefoot running is touted as great because it strengthens all the minor/stabilizing muscles of the foot and legs. Much like using free weights vs a machine. Not strengthening those muscles will lead to muscle imbalances and that's a recipe for overuse injury.
The mayfly has a 10mm heel-toe drop (25mm and 15mm stack heights) according to Runningwarehouse. So yes, the shoe they used does have a fair amount of cushioning. Sorry RC but it didn't take much
So naturally cushioned shoes (or really any shoe that has a full sole and interferes with truly barefoot running) will not incorporate those minor muscles fully. That means less energy is used during movement, which means greater efficiency in the short term.
I've always seen barefoot running as a great way to train and minimize overuse injuries, but for performance days you use a shoe with a bit of cushioning to give you a leg up. Long term use of a cushioned shoe will just result in detraining of those valuable little muscles and negate the benefits of their existence.
There is no "one camp is better than the other" in virtually any aspect of life. Both concepts have merits and applied appropriately a best of both worlds is almost always obtained.
Finally the article was pretty decent about limiting the scope. It may be metabolically more efficient to wear shoes but that may not add real world gains. For comparison, it's metabolically more efficient to not use trekking poles since incorporating your upper body uses a lot of energy. However if you have weaker legs and strong enough arms, trekking poles provide a real world benefit because you are not taxing your leg muscles as much, allowing you to move faster or longer. This is an issue of global measures and local variables.
Also when did "natural" become equated with "efficient." Climbing barefoot is natural, but climbing with a rock shoe is definitely more efficient. That's the point of technology, to increase our efficiency (at least short term).
Look at any high-performance entity. They all have a strong base that allows them to maximize potential. Navy SEALs have a ton of gadgetry and gear, but give them just a knife and they can still accomplish many of their missions. The top F1 drivers love to have a phenomenal vehicle, but their skills and training as drivers allows them to overcome gear failings. Having every muscle properly conditioned allows one to fully utilize the minor performance advantages that hi-tech clothing adds. Skurka and Ryan Jordan could probably outhike many of the people on this forum with SUL packs while they wear 30+lbs (I know they could me) because they have put in the time and training to strengthen what matters, gear is secondary.