Hmm - I'm not saying technique doesn't come into it. I'm just saying that it seems sensible to me to rule out poor physical mobility first.
In my own case, I think I could make a "full" shoulder turn, but right now I'd do it by overturning my hips. I don't think my hips are sliding. Of course, we could then argue about what makes for "over turned hips". But that's my point - I think the physical screens make for a good, objective measure of physical capability. And if I pass basic physical benchmarks, then I can worry about technique down the line.
What I do know is this. In the last 40-odd years, I've done close to zero golf-specific fitness work and very minimal stretching or flexibility work. I don't get to play enough golf for that to count. I have measurably poor flexibility for just about any activity, regardless of whether you see golf as an athletic endeavour or not. But I can't think of another sport where flexibility wouldn't count as part of your "fitness" to practice any part of the required technique. Heck, I have flexibility issues just riding a bike. So given what I know about my current level of agility - it seems sensible to me to check that avenue out before worrying about a bent left arm, or indeed many other positional niceties.
But for all I know, your typical students come to you with better flexibility than I currently enjoy - and it usually IS a technique issue. Again, that's what I see being the purpose and value of the screens.