The simple thing is this: up until about 25 years ago, investment banks were overwhelmingly populated by Ivy League grads who went to private high schools in the Northeast -- i.e., preppies. This cohort is still way overrepresented on the Street, but in the 80s it was literally like 70% of the warm bodies in bulge-bracket investment banks, hedge funds, and private equity firms. Thus, the fact that people on Wall Street still dress super-preppy is largely a function of the basic "dress like your boss" phenomenon. The people running large firms learned sartorial norms from their bosses in the 90s when they were analysts, and analysts today are learning from them.
Fashion groupthink pervades pretty much every career field. Military officers are keeping the braided leather belt industry in business through their demand for appropriate civilian attire; engineers in SV dress like slobs; people in the apparel industry wear tight black things. The observations in the essay strike me as a bunch of "so what".
At least he quoted Matt Levine, I'll give him that.