"Keep" is the operative word here.
If anything Woodward has his reputation to lose if you numbskulls manage to convince the rest of your drinking buddies Woodward is part of the "deep state".
Well, you've asked why no one sues. No one sues because to win a libel/slander suit, you must be able to demonstrate the dollar amount of the injuries you sustained due to the libelous/slanderous statements. The point in bringing up that this is an opinion piece, not a fact piece, would allow for Woodward's publisher's lawyers to demonstrate it is just an opinion, and that there is no monetary cost to the lie, thus negating the standing for the suit. Additionally, precedent has shown that people in the public eye have additional hurdles to meet than just Joe Shmoe citizen.
Woodward wrote an opinion piece, denied by all involved (not some, not most, ALL), at the end of his career which has been a one-hit wonder. It is worth no more or no less than that.