The Lexington site indicates that the reed switch is excited by 3 volts, which is the same as going to the potentiometer.
If it is a 10k pot, then 0.3 ma is drawn, and a 20k pot would only bring that down to 0.15 or 1/2 the current. And if the voltage is indeed excited by a pulse, then the draw will be less, but not sure how the ISS would know when to pulse it to catch all the revolutions of the wind cups if both the vane and cups are excited by the same supply line.
In general, it would seem that there were several ways to do direction. Grey encoding like the Heathkits do, with only 16 points of the compass, vs. a potentiometer, which itself has two easy ways to give direction. One is to give the potentiometer a known voltage, and if the pot is a known value, 'look' for a returned voltage that is referenced to a calibrated direction, that is at 180 degrees, if you sent 3 volts out to the pot, then you'd expect 1.5 volts back.
The other, somewhat more elegant way, is to not give a flying care about what the pot's resistance was (other than to take into account drain and damage if too much current goes through the pot) but to look at the ratio of the voltage coming back. If the pot is at 180 degrees and you send up 5.345 volts to a 16,750 ohm pot, then you get half the voltage back at 180 no matter what the potentiometer really is for resistance. This method seems a bit more self correcting.