I have an ex-Spetsnaz friend who trains specops (and any privateers with the $$), and he indeed does show people this style along with dozens of others. It's something that is useful in rare situations with certain weapons at certain ranges. Fwiw I have about 20 years of firearms experience, and a bit longer with more conventional martial arts experience (thai boxing, boxing, pankrate, bjj, etc), and this guy is someone I would not ever want as an enemy.
So, yes, horizontal firing grip is valid, but not for most people in most situations.