I don't think of every PhD barista as upper class at all. Holding a doctorate is at the upper end of academic attainment. Someone without even an undergraduate degree but who inherited wealth is not going to see that barista as their social peer.
* PhD barista: Low wealth, high class
* High-school educated trust fund baby: High wealth, low class
This sort of framing does seem very medieval though - I get mental images of rich merchants and indebted aristocrats when reading it.
To return to the topic of the article, a limo is (was) a means of signalling wealth. You cannot tell from the outside if the person riding in it has inherited their wealth or earned it from monetising their PhD. But the world changed and now you can see teenagers from working class families able to afford to rent one for the high school prom, or 20-somethings for a stag night.