It's a small step to say Oliver Cowdery was just expressing his opinion in Letter VII when he wrote that it was a fact that the Cumorah of Mormon 6:6 is the hill in western New York from which Joseph obtained the plates.
It's another small step to say Joseph Smith adopted a false tradition when he republished Letter VII and wrote D&C 128:20.
Then another small step to say the prophets who reaffirmed the New York Cumorah were merely expressing their private, incorrect opinions, including in General Conference.
Yet another step to say Joseph and Oliver were wrong about the translation of the Book of Mormon.
Another small steop to say Joseph never even used the plates.
And so it goes.
This is how persuasion works: