Yes, the fixed 100k ohm resistor would be seen as the input impedance.
However... in the case of the pot and a fixed resistor the input impedance is the series part of the pot (unless it is at maximum) and the parallel combination of the fixed resistor with the remaining part of the pot.
That would be a reason for R1 to be a very high value. It would make very little difference and the grid leak value will remain the value of the pot.
