The Wikipedia page for a sibling comment points out that Montreal already had problems: bank robberies, riots and terrorist bombings.
So if it's a 'natural experiment' it's one against a background of high levels of disorder to begin with.
I think it's also somewhat of a caricature to imagine that a total police strike is somehow indicative of what a radical reduction on police activity would look like. Most people calling for "de-policing" would draw the line at "let's not send police after bank robberies, assaults, riots, home invasions, etc".
You could hardly pick a more dangerous time or place in Canadian history.