It's worth keeping in mind that whatever you may think of the NRO gang, they're about three hundred times as intellectually honest as the average conservative broadcast media outlet.
He overstates the case, but there's some truth to it which I think rests mostly on the fact that in print or online your words have a degree of permanence which force you to at least moderate your tendencies towards hacktackular hypocrisy as others can easily call you on it.
On the radio you can largely say what you want without fear of anyone actually calling you on it. Sure, a couple of the biggest conservative talkers have liberal watchdogs like Media Matters paying attention and transcribing them, but there are dozens and dozens of little Limbaughs all over the country who say whatever the hell they want.