The argument here is in effect that no vernacular variant of any language could possibly embody a mistake. Which is true for some definitions of mistake, and false for others.
The problem with the definitions that make it true is that there's a slippery slope from dialects to idiolects. How many people have to speak a dialect for it to count as a dialect? You can't have any threshold, or you get weird consequences in hypothetical cases, like all the speakers of a dialect dying off except one. So if you decide that no vernacular can embody mistakes, you're bound to conclude that nothing any individual does can be a mistake, so long as they do it consistently.
The problem with the definitions that make it true is that there's a slippery slope from dialects to idiolects. How many people have to speak a dialect for it to count as a dialect? You can't have any threshold, or you get weird consequences in hypothetical cases, like all the speakers of a dialect dying off except one. So if you decide that no vernacular can embody mistakes, you're bound to conclude that nothing any individual does can be a mistake, so long as they do it consistently.