The deduction is flawed because the success of one method (thinking with writing) does not necessarily disprove the success of other methods (such as thinking without writing).
* Although it's not really right to use this kind of language here (premise, conclusion, deduction). It's a casual statement, so I suppose people can somewhat reasonably argue about it, but the assertion is tautological ('if something is incomplete, it isn't fully formed').
Similarly, if walking North always brings you closer to the North Pole, then you can never reach the North Pole without walking North, or at all. But look out for oceans.