Well for starters he's James T. Kirk, living legend. Decker is an "untried captain". Maybe an M-5 could find why Decker would be at an advantage.I agree with that interpretation. But by the question I mean, where's the specific evidence on screen? Decker's a completely new character, all we know about him is what's in the film, and we know Kirk was dead wrong in half of his statement there. Putting our bias for Kirk aside, where's the specific evidence that Kirk isn't overestimating or even exaggerating the significance of his experience next to Decker's?
More seriously, Decker is meant to be a man with serious potential, but he's clearly not supposed to be "there" yet. He is Will Riker, after all. Thankfully he is never the cartoon that John Harriman was.
I think Kirk's "unfamiliarity" with the ship is probably overstated. If he was being asked to fly the ship himself or rebalance the warp drive, sure he's not up on the new designs. That's why there are 499 people who know their jobs.