Although the treaty does seem to be pretty clearly prohibitionist, there is some debate (covered under "Possession for personal use", https://en.wikipedia.org/wiki/Single_Convention_on_Narcotic_...) as to whether or not the treaty requires member states to prohibit possession for personal use.

Is canada not a signatory on that treaty then?

That page doesn't mention marijuana. I don't think marijuana is classified as a narcotic, despite the vagueness of the definition.

Cannabis is explicitly listed as a Schedule I drug on that page.