Countersteering is convenient, but not required. A bicycle and rider are not a single rigid body. You can simply lean to one side and you'll have to steer in that direction to keep your bike under you -- no countersteer necessary.
I am not saying people don't countersteer, only that it isn't necessary to make a turn.
Also, bicycles aren't motorcycles where the weight ratio between rider and vehicle is swapped.
If you don't feel like clamping your handlebars so they only turn one direction, try this: coast along a straight line (and outdoor basketball court is great). Then pick a direction and just lean that way. You can absolutely keep your wheels on the line until you turn in the direction you picked, with no countersteering necessary.
This "fact" came about with a video of low skill riders who can't manipulate a bike very well, or don't know what it is they're doing when they do it.
He might be wrong, and just didn't know enough, but he is usually researching his videos very well and I would be surprised for him to be wrong about this.
What he demonstrated first of all is that you simply can't ride a bike that prevents the steer from turning in one direction. A bike continously falls to one side or the other, which gets corrected by steering in that direction to put the contact area back under the center of mass. This happens not necessarily through the rider's (conscious) action, even a bike with no rider does it to some extent, but in all cases it requires the ability to turn the wheel assembly freely around the steering axis.
IMO that means the bike in the video demonstrates failure to keep basic balance even before it gets a chance to demonstrate failure to turn properly.
just try it. roll perfectly straight and lean to a side. It accomplishes the same thing as countersteering, which is to put weight on one side of the bike, so when you turn that direction, the bike gets under you and you don't fall.
That's literally what they did in the video, I recommend you watch it. The issue is: you think you can just lean on the side you want, but you can't without counter-steering, so they devised a bike that literally does not allow counter-steering: you can't take a turn at all.
Because of the geometry of the front fork (rake and trail), if you lean to one side the bars will initially turn in the opposite direction. It's still countersteering. If you ever ride a bike where this geometry is wrong, which is difficult because it's so necessary, it's very different to ride, and you'll struggle to ride it non-handed.
I'm not sure your line method proves much since the countersteer could be very subtle. However I think you're still right. Countersteer makes you start to fall faster, but it clearly isn't necessary to make you fall because you do that anyway!
No it's not necessary in the strictest sense of the word, but counter steering achieves higher turn speeds, offers better control and precision, requires far less physical effort, and works at nearly every speed. I can't think of many pragmatic reasons not to use counter steering outside of just screwing around for fun.
This effect has a big play on motorcycles. Riding a bicycle is considered prerequisite knowledge for learning to ride a motorcycle, largely because of this counter steering. One thing that is more pronounced on a motorcycle is that counter steering only occurs while the bike is moving at speed. As in, you only counter steer a motorcycle above ~10mph (higher for some motorcycles). It's really cool to think about how intuitive this switch is, almost everyone picks it up quickly and it becomes second nature.
Countersteering applies at all speeds, it's just that balance plays a bigger role at lower speeds. The wikipedia article on countersteering goes into this a little bit.
Ride a motorcycle for 5 minutes and you'll believe.
EDIT: To answer your question more directly, you are steering in one direction to initiate a lean in the opposite direction. E.g. if you are attempting to turn right, you first steer left which generates force in a left-sided contact patch in the front tire, which causes the bike to lean right. The bike then assumes a stable right lean angle (you have to do some work with your body, but the bike naturally wants to do this), and the front wheel comes back into alignment, and you are now turning right.
Yeah, but you quickly counteract with direct steering after you initiate the turn, so the overriding sensation is one of direct steering at low speeds, even through the physics is the same. This is what I think most people are referring to when they talk about high speed / low speed steering.
The debate is whether countersteering is something you have to consciously do by turning the steering column ("yaw"), or whether it's an automatic effect of pushing/leaning down on the side you want to turn toward ("roll").
"countersteering isn't real" because "steering isn't real", cycles at speed turn by leaning/rolling, not steering/yawing.
Countersteering and roll are not mutually exclusive. The countersteer generally happens when steering whether you think about it or not. If you know about countersteering, you can practice it to make emergency turns.
For roll, Leaning in the direction of the bike is actually a bad habit (but it'll be fine on most road turns)
It turns out you want to lean the bike, and lean/shift your body in the opposite direction. This keeps center of mass above the wheels.
For example, the pro motorcycle racers with their knee an inch off the ground,they're leaning their body weight away from the turn, away from the ground. Meanwhile their bikes are leaning crazy hard.
That style of leaning is important for fast descents, or switchbacks, particularly switchbacks. Eg: "MOUNTAIN BIKE TIPS: CORNERING WITH CONFIDENCE" (start at 1:45) https://youtu.be/GFKPtEzE4xw
If you need to quickly swerve out of the way of an obstacle you push hard on the handle and you will immediately initiate a turn (you could just as easily say that your are initiating a lean). It's also well understood that handle bar input allows you to increase / adjust the lean mid-corner. I ride and never knew there was a debate about this.
It's pretty much impossible to believe without thinking it through, and yet everyone naturally intuits it.
It's one of my favourite examples of how the brain can just 'feel' forces and make the right adjustments incredibly fast. So amazing.