But that's exactly what I take issue with - you're not! Someone's delivering your food, someone's working at the bodega you go to. You won't encounter nearly the same level of homelessness as you do in SF, but you will encounter homeless people regularly nonetheless. You run into the actual underclass far more often than you encounter the extremely wealthy.
If you call yourself part of the underclass, either you're: Forgetting that they exist, or you're pretending that your economic situation is somehow roughly equivalent. I don't think you need to keep that on your mind every single second of the day, but at least be honest about it.
You are much closer to your deliveryman than to fat cats making multi-million dollar salaries on Wall Street. That's the comparison the Author was making.