That's bogus.
There's a "magnetron" under the table, no way around it. A magnetron is the "business end" of a microwave oven.
A cell phone transmits a maximum of 300mw, that's 3/10 of a watt. A home microwave oven emits on average 1000watts. Even with 8 phones, you have an omnidirectional power of 2.4 watts total, leaving almost nothing to the grains. It takes over 30 seconds for pop corn to start popping in a microwave oven where the antenna is designed to concentrate the microwave towards the food, where a cell phone use an omnidirectional antenna.
Don't try using a magnetron at home if it's been removed from a microwave: it could kill you! Not only the microwaves could really cook you, these things run at a voltage over 1000 volts and, unlike a tv tube, there's enough current to kill.