I'm actually reasonably sure Sorkin does understand marginal rates, but it's also the case that people who writes about this stuff frequently fail to explain it to readers in a way which seems to be designed to deliberately mislead. I'm not sure if poor Mason understands, but at his implicit hourly wage he can work about 5 more annual hours to keep his take home income constant.
...and as Jay points out in the comments over there, with basic deductions poor Mason isn't going to face any tax increase at all.
