I'm not entirely sure what it means that ground and +5v have a large input impedance, but it sounds like I won't destroy my board that way at least.
It means that a series resistor (at the Arduino input) won't end-up reducing your signal voltage.
The series resistor and the input impedance form a voltage divider where the voltage is proportional to the resistance (or impedance) ratio. With a 1K or 10K series resistor, essentially zero voltage is dropped across the series resistor and (essentially) all of the voltage is ends-up across the 100M Ohm Arduino input.
When you instead put the 2K resistor in series with the 10K pot, you are creating a 10/(10+2) voltage divider and reducing the voltage by about 17%.
The downside is that a high (total parallel) resistance makes the input more noise-sensitive. So although 1M Ohm would only loose about 1% of the voltage, I'd probably stick with 1K if you feel a series resistor is useful. (Stray EMI can generate a noise current in the input connections... Per Ohm's Law - With a given current, higher impedance means higher voltage.) There is also an input capacitance, which can cause issues when there is a high source impedance.
It's my understanding that the Arduino has internal protection diodes to protect it from voltage spikes. But, I have no idea what the current cabability of these diodes is.... I'm pretty sure if you connect a 12V power supply to an I/O pin, you can blow the protection diode. The series resistor could potentially limit the current and allow the protection diode to safely do it's job.