I would have a very hard time accepting it to be true. They don't even strongly make the claim that it is true, they are correlating areas of the brain associated with memory as growing larger, and they are associating a larger brain area with better cognition, but later in the article indicate that there are some areas of the brain which are associated with memory which actually shrink.

I think we should question research which is overwhelmingly against our common experience of life. My memory is absolutely shot when I am consuming weed regularly. It's not particularly subtle, it is noticeably worse. I suppose there is room for a situation in which it is worse while I am heavily using, but if I were to cease maybe it will rebound and settle at a point which is better than it would have been had I not consumed any cannabis... but I don't see any reason to believe this.