All evidence points to the contrary. The need of Uthman to canonize the text was due to the variation that existed. He ordered those variants destroyed, removing all evidence of their content. I even recall from college that the caliph ordered all verses, in any form, to be collected; some of the verse was from scraps of vellum, reed, antler, etc... If thousands of memorizers truly existed, such an exercise wouldn't be needed. He would only need to get a dozen of the people that memorized it to recite the exact verse again.