"And valuable assets are not left to rot in dungeons."

Lucky him. But he isn't gullible enough to trust Beleth's word that this will be done, once she has the intelligence in her hands. So he says, "I would be delighted to share my evidence with your fellows, at your leisure." Should she choose to take the notes and books that he has referenced up until now, he won't stop her; evidently, the source of his information regarding the infiltration is elsewhere.