I fully believe the scores that are being shot. On my very first attempt, without playing any practice, I shot a 69. During that round I suffered 5 very stupid bogey's, with putts that just creep around the edge of the cup, and missed out on at least 2 other birdies by just inches.
I've since shot a 68, with fairly similar number of irritating bogey's, but made the round by finishing with 2 birdies. I know that if I could just get my game together that fraction more I could maybe get close to the big boys. This course rewards accuracy with driving and approach shots. The moment you start exploring bunkers and the rough, it can become a real test.
As for "Master" players quitting multiplayer after some bad shots, you're correct that it happens. It is always hard to tell why a player leaves, but it always looks bad if they leave after a few bad shots. Last week I left a multiplayer game, after playing a couple of terrible holes on St. Andrews. It wasn't so much the bad score, as I don't care about my average, but the fact that there were a couple of players whose games were taking ages to load for most shots, which was much more frustrating than my errors. What you can't tell is that I completed the round in single player mode with a 76.