High functioning doesn't necessarily mean operating at 100% optimum "non alcoholic" peak performance. We are trying to define a High Functioning alchy, not a High Functioning non alchy.
It means you are functioning at a level with your alcoholism that is above those who have progressed further down the alcoholic spiral. Which is enough to convince most people you aren't alcoholic. Its normally enough to convince your self too.
People in the street with no experience of alcoholism, think alchies are the guys who end up drinking out of paper bags on a park bench.
But not many, if any started there on the bench.
Millions of alchies would have drunk top end booze for years .... if we don't do anything about arresting the problem, we end up sliding down the scale. Partially functioning alcoholics .... make it to work when they can, on the verge of getting sacked, wife's gone, finances are a mess, most people now suspect if not are 100% certain they are in trouble with booze .... your not drinking top shelf anymore either.
Its a slide down ... these days, with so much awareness of alcoholism & other addictions & with lots of different kinds of help readily available, not many of us hit the rock bottom of yesteryear. So we get off the slide before it ends on a park bench with $2 wine in a bag.