They didn’t do that.

They just used high sugar grapes like gordo blanco, the yeast would go dormant once the alcohol content had got to about 14% and there would still be lots of sugar left.

Now they ferment up to the level of alcohol they want, then refrigerate the must, which makes the yeast go dormant and form hard shells so it can be filtered and centrifuged out.

If you killed the yeast while still active, the cells will rupture and the wine will taste like marmite.

It was common in the new world to create “sweet wines” by adding pure alcohol to the must, which would cause the yeast to become dormant, and then filter it out. This was how “white port” was made. Since the invention of refrigerated brine jackets, this method has been abandoned.