This may take us a bit far afield from the actual case, and of course the analogy may not be perfect, but I'll try. The problem in the case of Wickard v. Filburn was that the price for wheat had plummeted due to a combination of overproduction and underconsumption, causing problems for American farmers. To correct this, Congress enacted wheat production caps aimed at stabilizing the price of wheat by limiting supply. A farmer in Ohio, Filburn, exceeded his production cap but used all of the excess wheat produced on his own farm as feed for livestock rather than selling it in interstate commerce. When he was prosecuted for exceeding the cap, he argued that limiting production of wheat for on-farm use was outside the scope of Congress's authority to regulate interstate commerce. The Supreme Court rejected that argument, holding that the aggregate effects of excess production even for local consumption had an effect on the interstate wheat market by driving down demand for wheat that otherwise would have been purchased on the interstate market. Therefore even local production and consumption of wheat fell within Congress's power to regulate under its Commerce Clause authority.
As I understood Joe's point, he was arguing that there's really no "activity" underlying Congress's authority to regulate in Wickard, because the real problem was not producing excess wheat but rather refraining from purchasing wheat on the interstate market-- i.e., commercial inactivity. And I can sort of see that point; he's right that, had the excess wheat been produced but not consumed, then the excess production would not have been tied to any decreased demand. Seen from that perspective, the act upheld in Wickard was essentially a mandate that wheat used for on-farm consumption must be purchased on the interstate market. Congress could, in theory, have done away with production caps altogether and simply mandated that every farmer must purchase a certain amount of wheat (estimated as roughly the amount that woud be consumed by the farm's commercial activities) on the interstate market, regardless of whether the farmer's needs were met through local production of wheat or not. That would have had the same stabilizing effect as the production quota and would have been a direct mandate to engage in a specific commercial transaction, much like the ACA insurance mandate. I'm not sure if Congress would have upheld such a provision or not-- like Justice Roberts, I tend to think there's a distinction between regulating activity (in the form of production caps) and regulating inactivity (in the form of mandating market participants to engage in commercial transactions that they otherwise would not) that imposes a real limit on Congress's Commerce Clause authority even if an economist would view the two as functionally equivalent. But let's say for the sake of argument that such a mandate would be upheld. Even if that's true, it still seems to me that there's an "activity" here-- it's the activity of engaging in the broader agricultural market that defines "farmers" as a class. Farmers are by definition active participants in the agriculture market in a way that the uninsured (despite arguments about what they may or may not do in the future) are not active participants in the health care market. For the Wickard analogy to work, I think Joe would have to argue that Congress could constitutionally mandate not only farmers, but all Americans to buy some quota of wheat in order to stabilize agriculture prices. And I have a hard time, admittedly on a mostly intuitive level, accepting that that kind of dictate would be within Congress's Commerce Clause power.
