Animals often get re-classified as scientists gain more information. For instance, when I was growing up, it was common knowledge that the giant panda was absolutely not a bear, despite appearances to the contrary ... and now science has established that it is.
As for polar bears, the fact that they can interbreed with other bears makes me wonder why anyone ever thought that they were related to minks instead. Maybe Palmer mistook a flippant remark made by someone else for fact, or was just being flippant himself.