No doubt Brigham ran with it, but how do think he duped the whole rest of the Quorum to lie about Joseph teaching it without them witnessing it for themselves that he taught it? The entire quorum said, "We know the prophet didn't teach it, but we'll lie for you President Young"? Even every single apostate member of the quorum claimed Joseph taught it.
Emma says he didn't, but his brother William and his own Mom say he did. His own First Presidency and the entire Nauvoo Stake High Council say he did.
Joseph's actions speak louder than words. He gave land deeds to many of his plural wives. Unless you can account for Joseph giving free land to teenage girls some other way? Are you maintaining that he was trying to make it look like he did? The transfer deeds are in his writing, documented while he was alive.

