The ruling isn’t that growing food yourself is interstate commerce, they said that growing food yourself reduces interstate commerce because then you need to buy less food from others.
Imagine there’s a law saying that regulating abortion is federal responsibility and the senate then uses this law as justification to ban demonstrations around abortion clinics. Would that mean that demonstrating next to an abortion clinic counts as abortion?
That's why subsequent courts walked it back. It was an obviously flawed ruling that produces preposterous results.
I mean, can the government prohibit dying (and throw your corpse in jail) because death means buying less food from others? Can the federal government throw you in jail for not having enough children because that reduces the amount spent on baby clothes?