From the Reuters article:

"Despite the widely held view that cannabis is benign, this research adds to previous work suggesting otherwise," said Ian Hamilton, a lecturer in mental health at Britain's York University.

There are several “handwaving” maneuvers in the paper, best summarized by this quote from lead author (PhD candidate):

"Yankey said were limitations in the way marijuana use was assessed -- including that researchers could not be sure whether people had used the drug continuously since they first tried it."

A direct quote from the paper shows this:

"Duration of marijuana use was estimated by subtracting participant’s age at marijuana use initiation from the age at the 2005 screen."

Then the conclusion begins with:

Marijuana use, especially prolonged years of use, may increase the risk of hypertension mortality…

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Should be clear now.