When a new infectious disease causes intense sneezing and coughing, wouldn't it be prudent to assume that's it's spread through the air, until proven otherwise? I'm not a doctor, but that seemed pretty obvious to me long before masks were being officially recommended.

I think they knew long before they made it public though, because Tam essentially said to Canadians: "There's no evidence that masks slow the spread of Covid-19, but if you happen to have any masks, please donate them to a healthcare facility, even though there's no evidence that they work". Tam's flip-flopping on the mask issue created a lot of distrust in the public, and made the eventual anti-mask movement a lot bigger than it had to be.