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The impedance at the ADC input is about 10k. (100k||9.31k)+1k Thats 100k parallel with 9k then add 1K. MicroChip is not talking about the resistance to the voltage source but the impedance. (from the ADC's point of view it looks like a 10k to 8.3 volts)Microchip say 10k max at the voltage of interest.
Inside the ADC there are input protection diodes. (to ground and to supply) There diodes can only take a small amount of current.
The diode on the outside limits the voltage to -0.7V. The internal diode limits the voltage to -0.3V. That puts 0.4 volts across the 1k resistor or 400uA.
The same story on the supply side.
Your scope has a input impedance of 1meg. Add a 10:1 probe with a 9meg resistor and the tip of the probe has a impedance of 10meg and a output of 1meg.