Differential probe input impedance 1Meg, but 50ohm oscilloscope mode recommended
The probe itself is an active device which has input and output, each one with its characteristics (given in the manual):

Impedance matching, like with a standard signal generator with 50 Ohm output, minimizes signal degradation caused by reflection.
Edit: also check the answer provided by JRE on why 50 Ohm is commonly chosen.