TVS diodes can protect from transient voltage spikes when used properly (low impedance to the input connector, properly chosen and with the proper supporting circuitry)

Zener diodes can temporarily protect an overvoltage too but you will need something to take all that current that is now going through it, making it pretty hot and likely to fry something

You can pair something like this with a positive temperature coefficient thermistor to act as a fuse (as it gets hotter the resistance goes up).

These circuits are tricky to tune right, take up space and can add significant cost to a product. So with a drive in consumer products to lower cost smaller things you may not see them everywhere.

perhaps a they meant a zener diode, which can be used to clamp a voltage? The downside being that it dissipates the excess energy across it's self.

> how would a loose cable cause a voltage spike?

From the top of my flu-y head, connecting and disconnecting an inductive load (like a motor) can cause spikes. However in USB land, that's pretty rare.

dc-dc step up devices use an oscillator to charge and discharge an inductor to achieve a certain voltage increase. They _could_ be noisy when starting up, or have transient load.