All coffee, tea, hot towels... any "water" product that is made ON the airplane has had the water drawn from the potable water tanks.
The exception to this is the planes that do not have potable water tanks! Such as the ATR, EMB, etc. The coffee on those planes comes from the station, and is put into coffee pots on the plane that plug into heating outlets in the galley. That brings about it's own issues, like how long has it been since that coffee has been made, how long has it been since that pot has been cleaned, etc!
Bottom line: the best coffee does NOT come from the airplane, at all. Get it at the Starbucks in the airport.