if I had to guess, they were going to randomly selected locations, looking at the sky through the PVC pipe (presumably straight up), and seeing if it was obstructed by tree canopy or not, and then comparing to whether or not the satellite said there was tree cover in that location

Exactly this. Specifically, how much error rates increased as you moved towards the treeline. The "am I looking straight up?" check was a washer hanging from the pipe by a piece of string.