If it's added before fermentation, it's still not in the final product, as it's been converted to alcohol. But you're right, champagne, even the best brands, has some sugar added after the fermentation, about 12g/l. It's part of the method. It's still a rather tiny amount if you consider that an orange juice without added sugar has about 90g/l (and you do drink it by the litre, as opposed to champagne...)