McEachran did me another favor when he dropped a memo on my desk that said only "P-E-O-P-L-E." I'd been writing too many thumbsuckers from my desk and he wanted me to get out and talk to real folks again.
—Brian O'Neill, "A newsman who knew how to make his point," Pittsburgh Post-Gazette, May 27, 2002

[Calvin Trillin] is not sure what he will write for The New Yorker this fall, after returning from his traditional summer-long sojourn. "I would like to write some things that take advantage of what I have learned. Stuff that has stuck in my head, so to speak. What editors call 'thumbsuckers' but writers call 'think pieces.' "
—Christian Williams, "No More Ribs," The Washington Post, May 19, 1982

Here's the first citation for the "journalist" sense:

Thumbsucker is the new term for pundit — from the Hindu for ''learned person'' — which Time magazine popularized to mean ''longheaded commentator''; surely such media biggies are a more deliberative breed of pencil pushers. —William Safire, "On Language," The New York Times, June 29, 1980