Fine.
role-playing game
–noun a game in which participants adopt the roles of imaginary characters in an adventure under the direction of a Game Master.
(Random House Unabridged Dictionary, 2006)
role-playing game
n. A game in which players assume the roles of characters and act out fantastical adventures, the outcomes of which are partially determined by chance, as by the roll of dice.
(The American Heritage Dictionary of the English Language, 4th Edition, 2006)
Which of these definitions does D&D 4th Edition not satisfy, and in what way?
If you cannot answer this question, you cannot say that 4E is not an RPG. And therefore, Clark's opinion is, in fact, wrong.
Sorry, but I do not thing that Clark's opinion was about the general definition of what a roleplaying is but rather about the specifics of "characters" as defined by the actual game* -bold emphasis above mine. The definition you provide does not address them, yet it seems to be a most important thing.
*for example characters living with a gamey behaviour world -regarding our standards- or characters that we can map our current notion of reality regarding probable behaviours in some probable circumstances.