Can we calculate the upper bound of needed power from some information from datasheet?
Is my thinking reasonable?


No, because maximum RF input power is 23 dBm. That's 200 mW and that is all the power you could ever harvest. You can never simultaneously draw 100 mA at an output voltage of 4.3 volts. Please also remember that harvesting is never 100% efficient so you should be thinking around a maximum of 100 mW.

Minimum operating power is quoted as -5 dBm and that is 0.316 mW or, if that power were terminated in 50 ohms, is a voltage of 126 mV RMS.