Load of rubbish!
I will always talk to a horse is I am approaching it from behind, I will tell a horse to move oner when I am mucking out in a stable, I will tell it to stand, to trot/canter/walk/halt on the lunge or if a green horse, when I am riding it.
A horse cannot see your body language when you are on top of it!
F something startles a horse I will not talk to it sympathetically, I will issue it a command and, as its leader it will obey that command.
I will correct a young horse's misbehaviour with the word "EH!" even of I am 20 yards away.
Voice might not be part of a horse's understanding but they can sure learn a variety of words.
I do agree that body language is very important but when you are amongst horses so is the voice.