I haven't been there for six years now, but one of the problems I worked on at Etsy was getting people to stop using their cart as a bookmarking tool. While I was there I worked on the functionality to fave an item as well as the functionality to add items to a List of favorites. There was another tool called Treasuries for this purpose that we phased out, so there were at least three systems that Etsy built to try to help people keep track of items they like without putting them in their cart. I know when we introduced Lists, several colleagues and I worked on that functionality for a few months before it ever saw the light of day. Even so, users continued to put things in their cart "so they wouldn't forget them". It was a very frustrating result; this was the exact behavior we were hoping that Lists would eliminate.
It wouldn't surprise me if it's true that the item really is in that many carts. I would also agree that it's not useful or accurate information for you, another shopper, since an Etsy user "having it in their cart" is in many cases not a very strong signal that they will purchase the item in question. Inflating the number intentionally would definitely be a dark pattern. If that number is intentionally inflated or is known to be inaccurate or fictional, dark pattern for sure.
Whether the current functionality qualifies as a dark pattern or not is a lot harder to judge. Is it poor design? Yes. For sure. Is it harmful to the user? Again, I think yes.
Is it a dark pattern if you design something poorly unintentionally? Does that term measure intent or impact? In a legal context I would expect the measure to be intent, so in that context this is probably not a dark pattern, but HN is not a court of law, so in this context... probably yes?