But the gap around the door might only be millimeters wide, but it is 30cm or more long. It is "both* dimensions that must be a long way below the wavelength t

But the gap around the door might only be millimeters wide, but it is 30cm or more long. It is "both* dimensions that must be a long way below the wavelength to keep microwaves in. Instead the door seal uses a technique called a quarter wave choke. It relies on reflecting back any microwaves wanting to escape, and by making incoming and reflected waves perfectly cancel, no power is transmitted. As OP said, even a small amount of soup dripped onto the seal and your microwaves will all start escaping.