This is the approximate input impedance of the ADC so you should drive it with a much lower impedance source to get good operation. Using a source impedance as high as R_ain will almost guarantee poor results.
This is a not a resistor you put in series with the input, as the title of the thread suggests.
Correct, it’s really a maximum, and by series resistor, it’s probably better described as the Thevenin equivalent series resistance of the source feeding the pin.