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.
Variations existed in the written form not in the recitation because diacritics (tashkeel) were not standardized. It's a common misconception that the Quran is memorized through the written text. This is not how it is transmitted. Look up Tajweed, which are the rules of pronunciation of the Quran.
Furthermore, memorizers of the Quran can trace the chain of narration back to Muhammad himself. So there couldn't be any forgery or manipulation. Uthman doesn't have the power to erase people's memories.
In Arabic language different dialects/variations have different punctuations and pronunciations which changes how a word is pronounced but not what the word is or what it means.
