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).
You're objecting to the premise, not the conclusion*. The deduction is valid for the premise (the part in the 'if'). Well, assuming you accept that an idea that can be "more complete" isn't "fully formed", but I'd say that's definitional.
* 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').
Or with writing about it. But there's an implicit "if you haven't already written about it". We might wonder what other implicit preconditions there are.
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.
