It is not a rule/section, it is a matter of constitutional interpretation in accordance with longstanding caselaw. It first arose in another prohibition context (alcohol, of course) in 1896. Ontario AG v. Canada AG, otherwise known as the Prohibition Case, was decided by the Privy Council (it was at that time, Canada's highest court).
https://www.bailii.org/uk/cases/UKPC/1896/1896_20.html
Short strokes: It is beyond provincial power to ban alcohol outright, though they can regulate its sale and even ban the sale of it (but not its possession or mere manufacture). It is not beyond the power of Parliament to ban possession of alcohol outright or its manufacture under the criminal power, but the extensive regulation of commerce within a particular province is the jurisdiction of the provinces, not the federal government.
Sound familiar? The current cannabis issue reaches the very heart of the same issue decided in 1896 in one baby step. It's on all fours.
Paramountcy: Where there is a conflict between the two, the Federal statute prevails.
Moreover, it is more than that, as unlike the sale of alcohol, the regulation of conduct within a private residence is the sine qua non of the criminal power. Given that Parliament has acted (and continues to act in relation to cannabis under the CDSA) it's beyond provincial jurisdiction to ban the home growing of it outright. What a person does in the privacy of their own residence is not the business of the state other than through the criminal law (though it might be the business of your landlord; however, the law of landlord and tenant is not constitutional law). And the criminal law in Canada is reserved exclusively to the federal government under the constitution. (There are minor provincial offences, but they may not conflict with the criminal power of Parliament. And Parliament made growing 4 plants an exception to the existing criminal law. So the feds have occupied the field.
And that will be that.
Anyways, the presumption of Quebec and Manitoba to ban homegrowing outright will die a fiery death on first contact with a Superior court - and the provinces know it.
So why are they doing it anyways? Because that way it is Trudeau's fault. Simply put: they don't care. The prohibitionists will not go down without a fight.
Depending on the poll you read, Quebec is either only just in favour of legalization or is slightly against it. When it comes to likely voters, they remain opposed as the demographics are clear that support/opposition is disproportionate among the young/older voter (and younger voters often cannot be bothered to vote). Plus, it is never a loss in Quebec to have its legislation overturned by the "bully" federal government, because Quebec.
In Manitoba, it's purely ideological opposition from the PC party in power there. The majority of the people of Manitoba support legalization (and home growing). The PCs in Manitoba don't care. Since when has public opposition ever stopped a Tory in Canada on its own?
Kudos