But TBH the biggest problem is fear of dropping it.
Try again tomorrow.
Ah well, that is the very paradox of slow speed turning. That fear will give you a tension and tightness in, well, in the whole of you! So your arms aren't free and easy at the elbow, your head is desperate to just stare in-front, your torso is stiff and won't loosen up .... it all just becomes hard work, stiff and your ability to flick the bike about (like the bloke in your you tube clip) suffers.
There's a faith that you have to find, it's a point of no return - it's a point where the bike
will fall if you suddenly have a change of heart. And so of course it's hard to learn that experience; that feeling on a big heavy bike, 'cos quite understandably you don't want to drop it. (And that's why I often bang the 'start small' drum).
So yes, you know all about the obvious clutch, brake, drive, look where you want to go stuff, but what you've got to experience, and to muscle memory, is that feeling of 'hang in the air' as your bike is perfectly balanced between falling over and driving upright.
And that just takes practise. But to get that feeling, you have to be relaxed and be able to freely move about on your bike.
So .... Don't just practise U turns. Practise being relaxed. In your carpark ride around standing up, stand on one leg, go round and round (figures of 8's .. whatever ..) with just one hand, giant slaloms with one hand, ride with no hands, etc etc. Being relaxed on a bike is key to virtually every handling skill you can think of, from knee down on the track, to a u turn in a side road.
I can't stress that enough, and that takes us full circle - you don't want to drop it, so you (quite naturally) are a bit tense, and that tension manifests itself in your arms and elbows etc, so the bike won't turn and flick, so you get tense ..... It's a vicious circle.
Practise being loose on a bike.