Even assuming it's true that the feds can effectively suppress interstate trade of marijuana only by suppressing intrastate cultivation and possession, it doesn't follow that they have the Constitutional authority to do so. There is no reason to think the Framers were unaware that placing limits on federal power would make permitted actions less effective than they might be in the absence of such limits.
I grant that in the early era of the Constitution, there would have been great arguments over intrusive federal laws based on the commerce clause, or anything else, for that matter. But the potential for such arguments does not usefully illuminate the commerce clause issue because intrusive federal laws about commerce would have been politically impossible due to the weakness of the federal government and physically impossible and unnecessary due to the rudimentary state of communications in that era.