Thank you for your question today, I look forward to assisting you. I have nearly 20 years of legal experience in various disciplines, including JAG.
Well, his statement is not a revelation of defamation. If he had said "I've been lying the last few months, saying that you are a danger to soldiers just to get you. I don't actually believe that to be true, but I wanted to harm your career" that would be admission of defamation. It admits a false statement of fact, to cause you harm.
He said that you haven't been allowed to deploy because he "knows" you are a hazard to soldiers. That is a statement of his opinion and not any sort of admission of defamation.
His behavior, if based on an actual belief that you are a danger to soldiers, even if inaccurate (which is your position) is actually legally justifiable. He's not making false official statements, because he believes them.
At best, you're looking at a form of harassment, which has to be resolved through perhaps an IG complaint or a Court of Inquiry through Article 138 of the UCMJ (not a criminal investigation, but a command investigation).
If you can allege that he's harassing you based on race, religion, gender or age, you can file an EEO complaint.
Unfortunately, it appears though that the normal recourse available to you has already been granted. Your improper bar and flag were reviewed and removed. Your command refused to do the referral NCOER, which means that the act never actually happened.
So again, based on these facts, you're going to be restricted to the IG and/or a Court of Inquiry complaint.