Raw engine power!
Note that the propeller on the XF-84H Thunderscreech did move at supersonic speed. There is nothing inherent in propellers which prevents their tips from moving faster than the speed of sound.
On the other hand, the big diameter of a prop requires proportionally more torque to keep the prop rotating against the drag from the supersonic tips. Thus, a fan engine requires less torque to reach supersonic tip speeds on the fan blades.
Also, the shroud of a turbofan engine helps a lot to make the noise from supersonic tips manageable. The XF-84H's noise made people literally sick.