If anyone happens to have a strip of carpet, a couple of weights and a section of tread you can test this in two seconds.
Cut two sections of tread, lets say a 2x1 and 4x1 section. Attach the piece of carpet to the board. Put the weighted tread sections (weight them both equally!) on the carpet board (evenly distributing the weight would be best, like a 1/2" piece of steel flatbar on one and 1/4" thick piece of flatbar behind the other. Tilt the board. Which one slides first? Argument solved.
Okay, well not solved, but better than a lot of the "he-said she-said" arguing going on here.
I'm very curious to see the "real" answer, since Cyberblue seems to be puzzled by their data.