Mr. Laser Beam wrote:
IIRC, Kirk does wear the insignia of a Commander when they're at the all-hands meeting with Marcus. It would have been the first time we ever see Kirk wear the insignia of any rank lower than Captain.
(For most of ST XI, Kirk is a Lieutenant, but does not wear a full Starfleet uniform.)
I'm pretty sure he's still in Captain's dress uniform, with three stripes, four pips and the TMP-style insignia set in a circle. A commander would have two stripes, three pips and a plain arrowhead (and Admiral a white uniform front and gold insignia set in a circle)
Since Kirk presumably walked right out of Starfleet after being bumped back to the academy, then went straight from the bar to the emergency meeting, he didn't have time to requisition a new uniform.