A bit cannot be well, harmful on purpose. (most anyways.)
In some ways it is like saying that bits hurt horses like guns kill people.
But the matter of truth is, people kill people.
In other words, it depends on what kind of hands you have. If you have hard hands, any bit you use, even the most softest its will be hard. If you have soft hands, then even the most harmful-causing bits will be soft. usually.
But anything can be painful or wrong in the wrong hands.
Guns kill people.
Spoons make people fat.
Pencils misspell words.
And bits hurt horses.
While I agree with this and the rest of you who say that it's a rider who hurts the horse, and not the tack, I would like to point out, that certain tack is much more prone to hurting a horse than others.
Yeah, guns don't kill people. People do. Guns can't fire by themselves, and that's a fact. However, this doesn't mean you should hand a gun to a kid who doesn't understand how the gun works or how to be safe with it. It is very easy to kill people with a gun, even unintentionally, so caution must be exercised when using and handling one, so only those who have been properly taught and are of sound mind should be able to shoot one.
It's kind of an extreme analogy, but bits work the same way. I will try to choose the tack and bit that is most comfortable for my horse and least likely to cause her discomfort or pain, so I reduce the risk of unintentionally hurting her. A horse in pain will not perform at their peak and can even become dangerous. I am a relatively experienced rider, and I have soft hands. I can ride nearly any horse in any bit without much of an issue as far as pissing the horse off or causing injury to the mouth, and I may even find that some different, and potentially harsh bits are effective tools for me and a specific riding goal, but I would not put a long-shanked curb or a twisted wire in the hands of a beginner, because if a rider does pull hard on the bit, certain kinds of bits are less likely to hurt the horse or cause the horse severe discomfort.
If all I want to do is shoot cans off a wall, I could use a rifle, or I could use a water gun. In the hands of a somewhat experienced shooter, either option would work safely, but by choosing to use the water gun instead of the rifle, you greatly reduce your risk of hurting someone, especially in the hands of a beginner. This is why I always recommend the mildest option to get the job done.
With that said, I would like to critique the video by pointing out that bitless is not always the kindest method or the most preferred by the horse. A rider must apply pressure somewhere to communicate to their horse and bitless bridles work because they apply pressure. The pressure is just not applied inside the mouth. It is instead applied to the bridge of the nose, or the poll, depending on the type of bitless bridle and these are also extremely sensitive areas on the horse's head. Some horses much prefer pressure applied to a well-fitted bit, over pressure applied to a thin strap over their nose. Your job, as a rider, is to find what works best for you and your horse, as not all horses have the same conformation inside the mouth and not all horses have the same preferences for where pressure is applied.
My own horse goes well in a single jointed snaffle with a curved mouth piece. She does not toss her head around, open her mouth or put her tongue over the bit. She eagerly accepts the bit while tacking up. Horses that are unhappy with their rider's choice of bit will try to tell their rider in one way or another. It is true that a rider does not always listen to their horse, and that is a sad reality, but many riders do listen to their horses and have chosen a bit or bitless bridle that their horse is comfortable with. Whatever has been chosen, bit or no bit, curb or snaffle, as long as the rider remains safely in control and the horse is not showing signs of discomfort, there is nothing wrong with that choice.