If they were interested in increasing sweetness, they likely would have been adding the honey after fermentation, not before. But either way, I suspect the supposition at the end of the article is the correct one, whether they were getting the sweetness from raisins or honey, the consumers wouldn't have cared. I went down an exploration of trying to approximate Roman wine a little while ago, and what I discovered reading a bunch about it (and subsequently creating and tasting the best replica I could), Roman wines were by and large some really nasty swill. They didn't fully understand how fermentation worked, the pine pitch the used to seal the amphoras leeched into the wines, and they typically attempted to hide the bad character of the wines by watering it down and then adding all kinds of sweet and spicy additives to it.