This is not a conspiracy theory. It is very carefully and lucidly written, with so many details, each of which can be refuted. How does he know about all the meetings between the CIA, Sullivan, etc. Why does no one refute individual facts?
I think he did have a source who provided all this. If the source lied, tough. Investigative journalism is always a gamble. If mainstream media worked, they'd try to press the government on the myriads of claims presented in Hersh's article. Perhaps this would lead somewhere. But the days when mainstream opposed the U.S. government like in the case of Abu Ghraib are over.
That's not how it works. The onus is on the journalist and the editors to ensure that any source they're relying on is credible, in a position to know what they're claiming, and not playing you. That's why most will insist on dual-sourcing any particularly sensational claims unless they really trust the source.
In the past journalists have been fired for relying on non-credible sources, most recently James LaPorta, so this is no small thing.
If Sy Hersh is not verifying anything about the source he's relying on, he's just being a transcriber not a journalist.