Is this way of sizing the load correct for this PNP output sensor?
How come 35mA supply is enough if the output can source 100mA?


The "35 mA or less" is the current taken by the device with no load on the PNP output.


  Does that mean the resistor/load between the sensor's PNP output and
  the GND should be more than 30V/100mA = 300 Ohm?


Yes, at 30 volts but if the supply that feeds the load is only 24 volts then the resistance of the load can drop to 240 ohms and still maintain current no more than 100 mA.