In theory, true.
Just like a good manager can manage anything.
In practice, however, a lobbyist is much more valuable if he or she has cultivated contacts and inside access to a particular regulatory bureaucracy. They guy pestering the Assistant Deputy Undersecretary in the lobby is vastly less effective, and commands much less money, than the guy who can dial the private phone number of the department head's own secretary and schedule a couple hours with his immediate successor in the job of department head.
And that's where the conflict of interest lives: a person gained access and personal trust in the context of public service. He cashes in on that asset, originally conferred for the benefit of the public, for his own personal benefit (bigtime lobbying contracts) and the benefit of his private clients (in the regulated field). Plus, you know, regulatory capture.