Preemptive caveats: I am not a dyed in the wool MFAer. And I am not as sartorially sophisticated (COINED IT) as most people on this sub.
Preppy is an affectation of class. When I think real class, I think subdued tones, mostly monochromatic individual articles of clothing (shirt? Solid light burgundy henley, buttoned up all the way. Layer? Solid charcoal woolen quarter-zip. Etc.), and subdued patterns (I can't stand checkers unless they're on a scarf, for example, and even then there's a part of me that's in pain). It's a very clean look, not only in terms of the style of clothing but the color (I think there's some overlap there but again I'm very new to this, sorry). It shouldn't distract from your facr and physique, should very gently accentuate your good features but otherwise stays the fuck out of the way.
When I think preppy, I think loud. Not quite as clean and simple, in terms of style or color. Khakis (I think they call them chinos here?), polo shirts, boat shoes, leather jackets, jeans that are weird ass colors, Chubby shorts, etc.
None of these things are bad individually, but together they constitute the preppy uniform. Honestly just swap two of the items out and it's a totally different style.
Basically, they're trying to look good, but they don't quite know what to do, so they seek safety in the form of a really formulaic approach to clothing.
I think he missed the mark completely, MFA dresses for the most part like hipsters imo. That's coming from an outsider - I've sort of hacked and slashed my way into a pretty visually/artistically conservative, WASPy wardrobe, just by picking out what I think fits me best, which are simple but nice clothes. I found MFA after I'd developed my style. I'm sure there are fault lines within the community which are very complicated and to which my generalization does a terrible injustice. But if someone from the 90% of the American population which is not into looking nice asked me to describe MFA's style, the best way I can think of to communicate it is "hipstery."