Your maths is basically correct, but likely it will use a 5amp or so fuse.
The fuse is there to prevent excess current in the event of a serious failure. If something inside shorts out, and it tries to draw 100 amps from the wall socket, that can damage wiring and even cause a fire.
So you have a fuse in the circuit, and once the current gets over say 5 amps, it's obvious something is wrong, and the fuse pops to prevent more damage. Melted wires / fires etc.. If you size a fuse too close to the actual draw you risk it randomly blowing due to thermal stress and in-rush currents, which is a nuisance.
But the fuse doesn't protect the UPS, It protects the rest of your wiring in case the UPS shorts out. If the fuse blows, it's probably because the UPS is seriously faulty.