While boycotting iPhones might be difficult, taking the Cabernet one shelf over takes almost no effort or negative impact to the consumer. It's rarely more expensive to switch from a US wine e.g. French or Australian. It's sold in the same store.
For the specific example of Sweden, every single wine bottle in every store is sorted by country in the store. And it sits above a price sticker with the flag of the origin country on it.
That's wildly different from similar choices e.g. when you pick a can of Hellman's Mayonnaise, there's no US flag under it. But for wine in particular, the country of origin is almost always known to the buyer and involved in the process, even if Sweden stands out as an extreme due to the flag being under the bottle.
That's why I believe the boycott which isn't a boycott on US wine but on US products in general, is _most notable_ for wine or in some other case not notable at all because it's a small effect and consumers don't always know what is American and what isn't (Hellman's example). But for wine, everyone knows.
The difference between the general slump in wine sales (10%) and the decline in US sales (20%) could have more factors, such as price. But it's easy to assume - and polls suggest - that consumers really do avoid the US wine.