Your not actually lowering the power, it's just lowering the duty time.
70% means that it runs only 70% of the time.
Microwaves are either on or off. There is no in-between.
Its the same as running something for 30 seconds letting it sit for 20 then running it for another 10.
It's also a large reason frozen dinners have you stir them.
With how powerful our military and air force is honestly it's more believable than not.