That's whatever the opposite of "appeal to authority" is. For starters there's a lot determined researchers can learn from declassified information about old ops and public information about current budgets and so on. And even if they worked in intelligence they wouldn't be able to publish anything classified.

What intrigues me is the claim "intelligence work is never useful". This is easily falsifiable with Wikipedia, which I assume even university professors deign to glance at on occasion. It's such a weird statement to make I feel like there's either more or GGP is mis-remembering something important from the book.