I would replace “seriously doubt whether Trump has anything like” with “I am certain that Trump does not have.” But yeah, Dowd knows Trump well and knew it was pointless to try to deliberately get the truth from him. But the truth does leak out of him in the form of projection, justifications, evasions, obsessive denials, and expressed fears, like the ones he leaked, if you’ll pardon the expression, to Comey about the pee tape’s existence.
If you examined him repeatedly on the same subjects at different times, you, as a lawyer, could get a pretty good idea of the threat environment based, not on what he says, but on what he denies, evades or lies about. And if you are stupid enough to take on the job of representing him, that’s your job. The problem is that he’ll fire you for doing it.