LeCun is very simply wrong in his argument here. His proof requires that all decoded tokens are conditionally independent, or at least that the chance of a wrong next token is independent. This is not the case.

Intuitively, some tokens are harder than others. There may be "crux" tokens in an output, after which the remaining tokens are substantially easier. It's also possible to recover from an incorrect token auto-regressively, by outputting tokens like "actually no..."