What blocks the radiation (at ~2.4GHz) is the metal all around the cooking cavity. The holes aren't actually a problem: as a rule of thumb we don't worry about shield leakage once the maximum dimension of a hole (diameter, in the case of a round hole) is less than 1/2 wavelength. 2.4GHz has a wavelength of 12.5 cm. Leakage does not happen through those holes.
(It does happen through the gap between the door and the rest of the microwave, but there's some neat design work to keep it from getting too bad.)
Of course it leaks, though. It leaks so much that when I was working on sensitive RF receivers, I could see when microwaves were turned on for lunch hour in other parts of the building. It's the leakage that makes it interfere with wifi. Don't worry though, you absorb more RF energy through the wifi in your devices and through your cell phone.
(It does happen through the gap between the door and the rest of the microwave, but there's some neat design work to keep it from getting too bad.)
Of course it leaks, though. It leaks so much that when I was working on sensitive RF receivers, I could see when microwaves were turned on for lunch hour in other parts of the building. It's the leakage that makes it interfere with wifi. Don't worry though, you absorb more RF energy through the wifi in your devices and through your cell phone.