That doesn't seem logical to me and I strongly doubt that this would be correct,
Why not? It's hardly contentious. Pretty well everything in life settles down to 3 classes, A are shagged whatever happens, B could go either way, C are sorted whatever happens. There isn't much point in spending lots of time working on A and C because very few of them will change groups, B is where it's at. All life is like that.
I cannot agree with your statement "Some of these people may be on the edge of having a problem" if you imagine this to be correct you can imagine all manner of things.
Whyever not? Populations are a continuum. There will always be people who can go either way - alcoholic vs not alcoholic, drug addict vs not, gay, bi or straight, pass 5 GCSEs or not, criminal or not, depending on the way life takes them. That's why Group B are so important, it's the old battlefield medical tent problem. A are dying regardless, so don't waste your time, give them morphine and a priest and wait for the inevitable, Group C are lightly wounded and they'll be fine regardless, so don't waste your time on them, the ones you are bothered about are Group B, the ones that might die or not depending upon what happens to them in the next half hour. These are your "might be on the edge" crowd.
"it's just a stealth tax". If it were about taxation then they'd just up duty. Duty on a bottle of wine is about £2.50, so the cheapest crap out there at £3 and £3.50 is actually worth 50p-£1. Up duty to £4.50 a bottle, the cheapest wines go to (say) £5-6, so the price basically doubles. The better wines at £10 and £20 go up to £12 and £22, a rise of sod-all. People buying those don't care about the £2, tax revenues would be greater, cheap wines would go up by the same amount as min alcohol pricing. Raising duty is easier than passing a new law anyway, the Chancellor just waits for the budget and says "2p on beer, 10p on Scotch, 20p on a packet of fags, STFU".