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):
Father Hugh thinks it would have a tremendous effect on simple, inexperienced people like televiewers; he thinks they would say, 'I must join the Church, if that's what its services are like.' Mind you, I wouldn't let him take part, he'd put in far too much Latin and worry people. For all he says he isn't, he's a bit of an ultramontane, in practice though not in theory, and we can't have that in the Church of England, we must stay dyed-in-the-wool Anglican.
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):
Centuries later, the nature of Ptolemy's theoretical system is fairly clear. Seven τονοι provided melodies which, like the ‘αρμονιαι, differed in character; and these τονοι were modes, being in essence species of the octave. Ptolemy offers no information about the relative values of their notes; for all he says we might believe that Mese (κατα δυναμιν) acted as tonic in each of them.
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):
Let us begin, then, with the analysis of sense perception that Moore offers in "Refutation of Idealism" in opposition to what he takes to be the Idealists' account. All sensations, he says, involve two terms: a respect in which they are alike, which he calls "consciousness" or, later, "awareness" and a respect in which they differ, which he calls the "object." For example, an awareness of a blue sky and an awareness of a green meadow are alike in being awarenesses but differ in their objects. Of course, Moore does not speak of blue skies and green meadows, he speaks merely of blue and of green, and he means, I think, that we are aware of the property blue. On the other hand, in his reply to Ducasse in the Schilpp volume (Schilpp 1942), Moore tells us that to say that blue exists is to say that some object that is blue exists; so, to be aware of blue may be to be aware of some blue object, and for all he says in “The Refutation of Idealism" that object might be the sky.