First off, he is talking about microwave radiation. Not like plutonium type of radioactive radiation. That's the same kind of radiation put out by your VHF radio, your radar, or any other transmitter, just a different frequency.
Then, the only real risk was from being harmed by the exploding appliances. There was practically no risk from the microwave radiation, as he contained and directed it using the tin can. Yes, a tin can is able to very effectively direct microwaves. He wasn't pointing it at himself, or anyone else, that I saw, which definitely would have been dangerous.
I would not call this any kind of stupidity at all. Rather an interesting demonstration of how microwaves can be concentrated and directed, and the effects when they are.