As an attorney, I'd say it was a poor defense, but sure, I suppose he could, in the same way he could say all sorts of things. Folks have tried that and failed time and time again. All the interstate commerce clause does is give Congress the power to regulate interstate commerce--it is not a defense someone can make when charged with a state crime. States have what is called the police power, allowing them to regulate matters within their borders for reasons of public safety, morals, etc.--a very large grant of power. The courts, both state and federal, have routinely said that this allows them to restrict entry of firearms, alcohol, fireworks, etc.
Yes, of course the chance of someone being charged with crossing a border with homebrews is small. But let's get the law right so people can make informed decisions for themselves.
