[gregates]

Stressing the "always" makes the argument valid only because it's a wordier version of "ideas can always be made more precise and complete, therefore no idea is perfectly precise and complete," which has nothing to do with writing. If we try to salvage the argument by making the assumption that the author obviously meant some ideas are perfect, but only written ideas, this becomes "unwrittendown ideas can always be made more precise and complete, therefore no unwrittendown idea is perfect". Which is vacuously valid in that the antecedent and consequent are identical.

The argument is either merely asserting the conclusion or invalid. I guess it's a matter of judgment which one is the charitable interpretation of the author's meaning.

Perhaps the most charitable interpretation is that the quoted bit isn't intended as an argument at all, just a restatement to cast an already-established conclusion in a different light. It's presented as a "shocking" additional implication, but perhaps it's the shock that's supposed to be novel, not the implication.