Decker was the captain in command of the vessel and its crew during its refit. As I understand it, that's not unusual in the navy. After all, even a ship undergoing a drydock refit has a crew that has responsibilities and needs a commanding officer to direct their efforts. Just because that isn't as exciting or glamorous as a captain in the field, that doesn't mean he wasn't a real captain.
But in a case like this, it's far easier to assume that the speaker was merely incorrect. Some people seem to want every single line of spoken dialogue to be treated as absolutely reliable gospel, but that's deeply unrealistic. In real life, people get things wrong all the time. That's why hearsay isn't admissible in court. So if a character asserts something that conflicts with canon, and there's nothing beyond that single assertion to back it up, the simplest, most likely explanation is that the character was wrong. Because if there's one thing more completely fanciful and unbelievable than a reality where people can travel through time, it's a reality where people never make mistakes.
My solution for in universe Canon mistakes? Time travel. There have been so many time travel events we've seen, and likely so many more that we haven't seen, that there is no real timeline. Everytime someone goes back BOOM, butterflies out the wazoo changing everything in the show.
But in a case like this, it's far easier to assume that the speaker was merely incorrect. Some people seem to want every single line of spoken dialogue to be treated as absolutely reliable gospel, but that's deeply unrealistic. In real life, people get things wrong all the time. That's why hearsay isn't admissible in court. So if a character asserts something that conflicts with canon, and there's nothing beyond that single assertion to back it up, the simplest, most likely explanation is that the character was wrong. Because if there's one thing more completely fanciful and unbelievable than a reality where people can travel through time, it's a reality where people never make mistakes.