With a great putting day, these scores are within the realm of possible. In one of our practice rounds, the -7 that Cerino mentioned, I played near-perfectly tee-to-green for the first 4 holes but didn't sink any of those makeable putts. Add another couple of 10 ft misses on the back 9 and I could easily have had 6 extra birdies to go with the 4 I did make.
Obviously a single-player score of -9 would require a lot of luck too, especially with the wind. Having outside chances of birdie on #13, #15, #17, and #18, rather than a tough fight to save par on each of those, would also make a huge difference.
RSG is unusual among our unevens starters courses in that pretty much every hole is potentially birdieable.