I think anyone who is able to decipher Sasori's poison as well as being one of the few medical ninjas in the war doing their job as efficiently as she is, is good enough to learn the technique. And even if she can't or hasn't yet, that doesn't mean she's not a good medical ninja, which seems to me that you're implying.
I think you misunderstood Tsunade. While it may have sounded like other medical ninja learned Byakugou, by saying "those" which is plural. She only really meant herself as Byakugou is an enhanced form of Creation Rebirth, a technique she has only been shown using. Basically she is saying she is the "only" exception to the rules.