Cuz Off A is still under legal threat...
Seems pretty clear that "Official A" is still under legal threat. Given that Fitz has had a new grand jury empaneled and will continue the investigation, it's way too early to say that this is "only" about Libby.
Given the job title (an Undersecretary of State) and what else we knew about which Undersecretary was in a position to know what and talk to whom, "A" seems very likely to be John Bolton, btw.