I, indeed, did find it interesting and to which I respond, "'dafuq?" I grew up in the U. S., and growing up actually raised sheep. And when you are served adult sheep, you are served "mutton". IOW, the term "mutton" was not "uncommon in the United States" in my part of the country at the time I was growing up.
But that's another time and another place; I'm kind of curious about when the change came about. Partly because, were that article written thirty years ago I would have argued that it is wrong. But I've since become a vegetarian, and might not have gotten the memo when the change came about. Or maybe I've always been wrong, along with all the other hicks I grew up with. :-)