At this point, you are required to sign two documents. One states that you will never reveal your employment with the agency even to family. The other states that you have no right to know the reasons if your employment is ever terminated or not ultimately formalized.
Unless you are ultimately interviewed at Langley, any employment offer is bogus.
So what happened? Did you not actually get the offer or did you turn it down?
Actually it depends on the job you are interviewing for.
And plenty that have signed the confidentiality agreements, upon the ending of their job, are then allowed to use on their resumes and tell faily they worked there. Not specifics, but acknowledgement and vague general terms.
It just depends what you did.
“One states that you will never reveal your employment with the agency even to family.”
You must have been going after a special position.