There have been several instances of the Koch Brother's attempting to silence critics through cease-and-desist letters from their in-house lawyers. Glenn Greenwald is one of the latest.
These tactics are empty threats. The last thing the Koch's want is to be deposed. If they actually tried to sue one of their critics, the defense has the legal right to depose the brothers before trial.
Depositions tend to be free-wheeling affairs with nothing off the table. I doubt they are willing to be forced to actually be put under oath and have defense attorneys grill them about the very thing they are trying to hide with their intimidation.
So long as anything written about them is the truth, nobody need fear their in-house attorneys no matter how intimidating their letters may be.