The problem is that the terms correct and incorrect are used in situations where non-standard, unusual or unlikely would be better descriptions.
Double negatives are non-standard but "correct" in certain dialects. The phrase a strong smoker is an unusual collocation (cf. a heavy smoker), but can hardly said to be incorrect. How have you got that black eye? is an unlikely sentence, but again cannot said to be incorrect.
Nevertheless, the term incorrect can be applied to large areas of grammar. It is simply incorrect to say: Understand do I you not or She has two dog or I will play football yesterday.
There's a case for saying that the authority in these cases is the grammatical hard-wiring that would lead every native English speaker to reject such formulations.
In an 1855 edition of "The Church Porch," edited by the Reverend Robert Willmott (incumbent of Bear Wood), the editor reports that the line
And the sad man is cock of all his jests.
actually means
The serious man wins the victory.
To reach that conclusion, one must first recognize that the stanza is comparing the virtues of a solemn or sober ("sad wise") valour to a giggling flightiness. The giggler begins to look bad from the moment "infection" or "a fir'd beacon" frightens him into silence—at which point, in the eyes of the assembled company, he becomes the object of scorn and derisive jest. Thus the serious ("sad") man triumphs ("is cock of all his jests").
Confirming this view is a footnote to the same stanza by Ascott Hope, A Book of Boyhoods (1882), reporting of the word sad in the first and last lines of the stanza,
This word then [in Herbert's time] implied serious, sober.
In a 1905 edition of Herbert's works, the editor, George Palmer, reads the final line of the stanza a bit more demonstratively:
It is the serious person who can crow at the end of the merriment. "He laughs best who laughs last."
So rather than meaning "The sad man is the butt of all his [the giggler's] jests," the final line means very nearly the opposite: "In the end, the serious man becomes the strutting (or crowing) champion of the biggest celebration."
Incidentally, Graham Greene used the line "And the sad man is cock of all his jests" as the epigraph to his novel Our Man in Havana (1958).
Best Answer
There are actually two ways to understand "For all Clark says" in the OP's example. One is the interpretation that chasly from uk and Brian Donovan give of it: "For all Clark says" means something like "Notwithstanding all the information that Clark provides." A second (and contrary) interpretation sees "For all Clark says" as being more akin to the idiomatic expression "For all we know," which implies "Given the limited knowledge we possess." In the OP's example, this second interpretation might work out as "Given the [incomplete] account that Clark offers."
The two meanings disagree on the question of whether the conclusion that follows the opening "For all Clark says" represents a result contrary to Clark's assertions (the position that the first interpretation above takes) or represents a result—of unspecified plausibility—that is not incompatible with Clark's commentary (the position that the second interpretation above takes).
An example where the first interpretation of the phrase "For all he says" seems clearly correct appears in Rose Macaulay, The Towers of Trebizond (2003):
An example where the second interpretation of the phrase "For all he says" seems correct appears in R.P. Winnington-Ingram, Mode in Ancient Greek Music (2015):
And an example where either interpretation might be valid appears in Ruth Anna Putnam, "Perception: From Moore to Austin" in The Story of Analytic Philosophy: Plot and Heroes (2002):