Every microwave oven on the planet uses a a device called a “magnetron” to generate its high-frequency electromagnetic waves. I won't go into how it does this, but the important thing to note is that the great old magnetron is one of the last holdouts of vacuum tube technology in current tech. Magnetrons are big, hot, dangerous, and power-hungry beast. The whole lot of them. A USB-powered device, necessarily, can not really be any of those things.

Now, being an engineer, I know that we can do shifty things like store all the necessary energy in a capacitor and blow it all at once into a magnetron for a quick burst of heating. However, 750ms worth of nuking isn't going to get your beans anywhere near hot, and meanwhile your microwave has turned into a molten pile of slag right there on your desk.

If this isn't shenanigans, it's at the very least the product of a trigger-happy marketing dweeb who doesn't know anything about microwaves other than the fact that they make beans hot.