"The researchers found that both recent and heavy lifetime cannabis use was associated with a statistically significant reduction in brain activity only during the working memory task."
They theorized that abstention may help long term users, but that wasn't in write-up
Also this is not necessarily a universally bad thing.
For example if I need to work through something from first principles as opposed to instant recall I may develop a more novel understanding.
Anecdotally I have found a number of individuals that are great a remembering and therefore excel in some academic situations are the quite intellectual rigid and unable to think beyond what they are told.
But as a disclaimer I'm not advocating marijuana use as it can have detrimental affects on motivation for example.
You are misunderstanding the meaning of working memory. Working memory is very short-term and small; it's what you use when you read the second half of an equation and still remember what the first half said.
A reduction in working memory is a direct reduction in cognitive capability for some/many tasks.
"The researchers found that both recent and heavy lifetime cannabis use was associated with a statistically significant reduction in brain activity only during the working memory task."
They theorized that abstention may help long term users, but that wasn't in write-up