That is not what the article is saying. The article is saying that pot decreases the circulation to vital areas of your brain enough that it can be be quantized in an MRI scan. And in fact it has a much larger effect on the brain's blood supply than any other external factor that they could find... nearly five times that of alcohol. Making light of this and ignoring this is irresponsible. Your arguments are juvenile to say the least and with this knowledge pushing for legislation to make marijuana more accessible is irresponsible.
That is not what the article is saying.
Yes it is: "Smoking cannabis ages the brain by an average of 2.8 years". And so does the research paper: "A regression model in which brain SPECT regions of interest was used to predict chronological age (CA) was then utilized to derive brain estimated age (BEA). The difference between CA and BEA was calculated to determine increased brain aging" (https://content.iospress.com/articles/journal-of-alzheimers-disease/jad180598)
Correlation does not prove causation; it could well be that pot smokers tend to more often engage in some other unhealthy activity that reduces circulation - or that some pre-existing condition predisposes one to both pot use and reduced circulation.