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').
* 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').