Well to be fair you have already suggested that has greater validity by virtue of being included in the "series proper". Which is a reasonable point, though I don't believe it is conclusive.
On the other hand, while, as M'Sharak points out, "Kirk as a stack of books on legs" is not, as far a I know, supported by anything we actually see, neither I believe is it contradicted.
But why is it so important that Kirk be that "stack of books with legs," when it had not been so throughout Shatner's non-WNMHGB career (78 live-action episodes, 22 animated episodes, 7 feature films) portraying Kirk?
That was the question: why has it become important that Kirk be measured against that particular yardstick now, when it never had been before? Why has it become necessary that we see this "stack of books with legs" in any situation other than the one
situation (even allowing Mitchell room for a bit of a fond exaggeration in reminiscence) in which it was supposed to have been true?
there was a reasonable expectation that one of the remaining films would be showing us Lieutenant Kirk, hard-nosed instructor of a "think or sink" course at Starfleet Academy, then yes, I might be looking for that as a prominent aspect of the characterization. But what's the likelihood of our seeing that on the big screen, really, besides practically nil?