That is correct, most microwaves make great faraday cages! problem... the power cable, it should be an old microwave, you must cut the power cord on it, it will act as a perfect antenna and induce electrical energy into the microwave during an event. If you want a workable microwave after such an event, go find a cheap one with a mechanical dial timer, they're still out there. The only function that will break on a microwave due to a solar storm or EMP will be the electronic timers. Which will obviously render the unit un-usable.
If you still have generator / solar / battery back up power, a microwave could prove to be a valuable commodity, no gas, no propane, worth having one!