Is every we think we know about corning wrong?
well.. no trail braking is a racing technique that is used on a track
the guy is saying things like "what if you go into a corner you are not familiar with and you judged it wrong and its tighter than you thought..." and a few other variations on that hypothetical
the answer is: you are riding beyond your skill level and you need to learn to judge a corner better before you fly into it and realize you are going too fast.
The reason trail braking is a track technique is because riders who race get to the track a week early, and ride the track hundreds of times, pushing themselves faster and faster in every curve, braking later and later, so they are riding around every curve at the very limit of traction, on the edge of dropping the bike... because they are racing, and if you are not going as fast as possible, someone will pass you and you lose.
The limit of how fast you can take any random curve is the limit of the friction of the tires on the pavement. If you trail brake into the turn you do load up the front tire, but the fact remains: you have about 1g max of friction on the road, if you are using part of that braking to load up the front tire, you cannot use it to keep the bike from sliding out sideways. And if you are shifting the weight of the bike to the front tire using the front brake, so the front tire has more downward force, then you are unloading the rear tire, and run the risk of IT losing enough downward force so that it breaks lose, and whips around on you. This is especially a risk if there is a bit of a bump in the road in the curve, having the back come around.
And if you are riding on unfamiliar roads taking the corners at the limits of traction, its just a matter of time before you are in the guard rail or in the woods, or in to the side of a pickup truck backing out of a driveway 3/4 thru the blind curve. Then you will have the joy of knowing you crashed going as fast as possible.
BTW, the shot of the guy about a minute in locking up his rear tire and hitting the guard rail, is an excellent demonstration that, when you lock up the rear tire you MUST hold it locked until the bike comes to a stop. If you let off the rear bike the rear tire snaps back in and throws you off the bike. I watched it several times pausing the video frame by frame, and you can see the spokes on the rear tire stop, and just before he is thrown off they start rotating again. He was not steering into the skid, he let off the rear brake and it threw him off, and into the guard rail.