I agree, this warrants a complaint. And I would argue because of, not in spite of, the fact that the individual concerned is in line for a promotion.
This would have been so much better handled had the crew member had a quiet word and said "I'm sorry, I'm not supposed to allow you in the business cabin, I need to ask you to return to your cabin" which would have allowed you the opportunity to explain the situation, and the response would (hopefully) have been "of course, just nip through the galley and return to your cabin"
IMHO, one of the key characteristics of a good SCCM/CSM/CSD is the ability to exercise a little discretion and flexibility. Someone who can look at a situation, know what is "supposed" to happen versus what "should" happen and make a judgment call is worth their weight in gold, and probably leads a good team and service as a result. An authoritarian, on the other hand, isn't good for passengers, and I suspect isn't good for the crew who work under them either, and probably (although I can't prove it) leads to some of the instances of poorer service we sometimes hear about here.