Why would they invent such a thing? They have plenty of evidence to put him away for narcotics anyway. Plus, this guy doesn't live in Yemen, he isn't going to get drone striked. He lives in San Francisco and will go before a judge. If his defence says, "we'd like to see the evidence of this alleged conversation" and the prosecution say, "uhhh well your honour, we sort of made that whole thing up" then the case collapses and the guy walks.

They aren't going to risk their entire case to invent evidence they don't need.

DPR was a guy who talked a good talk, and suckered a lot of people into believing he was some kind of saintly martyr. In fact he was the kind of guy who orders assassinations. That isn't so unbelievable if you read the rest of what he wrote.