These "Mormon offshoots" are members of the Fundamental Church of Jesus Christ of Latter-Day Saints which has nothing to do with the mainstream Church of Jesus Christ of Latter-Day Saints (LDS Church) and, yes, they still practice polygamy. When people call them Mormons they propagate the misconception that the mainstream religion continues to practice polygamy. It was practiced to a limited extent up to 120 years ago by the mainstream religion but the practice was discontinued completely and anyone practicing it excommunicated as they are today. These fundamentalists are certainly NOT Mormons.
Indeed, in the definitive sense, they are not Mormons, but they DO have Mormon roots or LDS roots I should say. I think I did read something where only about 3% of the mainstream religion practiced before they renounced it.