Yes that poly fuse is in the USB 5V supply and is designed to protect the USB power supply in the PC. They are part of the USB spec.
It is not designed to, nor will it, protect the circuits beyond the fuse. It will also not even be in the circuit while powering the arduino from an external power supply, the purpose of this thread I think.
As I said a poly fuse takes a long time to blow, sometimes as long as a second, depending on the degree of overload and will only protect components that are damaged by prolonged exposure to excessive power dissipation. It will not protect components from peak excessive current.