Arthur knows that Joan didn't compromise her identity with a comparable revelation that she works for CIA to her contact. Old habits die hard and inter-agency cooperation is rare. So if he did know she was CIA the offer to share information likely wouldn't have been given. The positive side of his being on the island for six years, which Arthur actually wouldn't want to think about, is that he hadn't been made head of his Division at that time so the name Campbell wouldn't have been familiar in that way.

"That sounds promising. I would imagine that they would have quite a lot to observe in the six years."