I’m not denying that this is the case so much as asking why it’s the case. For instance, we pair sweet wines and sweet food, and the sweet wines taste balanced whereas on its own it might taste overly sweet. The same tends to work for acidic wine and food (ie Italian red wine often high acid and tomato base sauces). Are the tannins that make chocolate bitter different in a way that sugars or acids aren’t? Would be curious in the science here.
I really don’t know the chemistry, but the thing is that some elements in wine vs. food pile up (like bitterness, spice, astringency), whereas some elements get compared to each other (well, mainly sweetness and acidity). A sweet food and sweet wine don’t taste twice as sweet together, but instead the sweetness from the food subtracts the sweetness from the wine. The same thing happens with acidity - higher acid foods diminish the sense of acidity in wine.
However, you have bitter elements in your food, they tend to only add on top of bitter elements in the wine. Some elements boost wine’s astringency further as well. I’ve no idea why these things happen, might be something to do with the texture and how tongue and the rest of the mouth translate tastes and flavors.
It’s an additive effect and happens with many different substances, short chain tannins being one of them.
Another common one is cooling found in toothpaste, mouthwash, chewing gum etc. Menthol by itself is cooling, combine it with any artificial Cooling molecules (WS 3, WS 23 etc) and the effect is much greater in either impact, lasting quality or both.
It may appear to be synergistic but the effect is mainly additive.
I really only find this to be the case with really dark chocolate. I tend not to find milk or bitter sweet chocolate to have the same issues.
Tannin reacts with proteins in saliva that make your mouth feel slippery, which is why your mouth gets progressively drier as you drink more young red wine. I imagine the chocolate tannins help use up the available saliva proteins.
Also, if the chocolate is sweet, that will accentuate the bitterness of any wine you drink alongside it, I think.
That said, I’ve enjoyed chocolate desserts with mature cabernets and Bordeauxs whose tannins have largely resolved. Gerald Asher wrote years ago that chocolate and claret was a common combo in the 19th century.