But we never actually see Decker with captain's stripes, only commander. So what conclusions can we draw? I think Decker wasn't officially a captain yet. He was involved in Enterprise's refit, and he had basically been tapped to be captain following Kirk's recommendation of him to that role, but the papers hadn't quite gone through yet.
An interesting idea but not actually true. And the proof is in TMP itself. When Decker makes his first appearance, he's wearing one of the alternate jumpsuits that has the rank epaulets on the shoulders
rather than at the wrists
. He's still
wearing this uniform when he meets Kirk in the corridor just after the transporter accident. Kirk wears a similar
uniform near the film's end. Both uniforms clearly indicate the rank of captain. It's not clear when Decker was actually promoted, but he was clearly a captain by 2273.