In other words, this "Mormon Offshoot" actually started centuries before the Mormon church was founded.
Maybe there's a different definition of "offshoot"......
It's just always been that way.
My own thoughts are that Smith stuffed his COFB materials into the Doctrines and Covenants just as filler but he'd had them around for a while. Later Mormons felt it necessary to interpret that material in terms of the LDS belief structure.
One of the mysteries is why he didn't also pick up on the late 1700 prophecies by the Two Sisters, and where's the story about cutting the horse loose ~ which is similar to the 5 nations story but the horse goes West and not East.
Eventually somebody will drop something on the net that explains those differences.