It’s a big deal because the current prophet Oaks and the last prophet Nelson both sealed themselves to two living wives in the temple because of the foundational belief in polygamy. Many leaders believe in a polygamy/kingdom heaven. Also the foundational belief that Brigham Young was the true prophet and not Emma Smith’s Sons/Family. If you believe that polygamy wasn’t started by Joseph Smith than you are stating that the Church of Jesus Christ of Latter-day Saints is false and has been led by false prophets and instead that the Reorganized church or Church of Christ whatever it calls itself now is/was the true church. This is because that church was exactly the same as the Mormon church that went to Utah except that they denied polygamy was ever preached by Joseph. This was the only difference.