Couldn’t have been terribly more potent than present wines. Maximum alcohol content without distillation is around 16%.
Higher concentrations the alcohol, which is essentially yeast urine, kills the yeast.
Modern strains of champagne yeast will ferment to around 22%, and are used to make super-strong beers.
One of my favorite wines is the Dobra Zemlja Zinfandel, which rings in at a stout 16.8% alcohol. It is so flavorful that most folks don’t even taste the alcohol...really good Zin.