Earlier quoted context omitted.
In the "THe VILLaIN aRC oF VANiTY SiZINg" section, vanity sizing is framed as marketing strategy which is successful because of the psychology around that - linking out to https://www.sciencedirect.com/science/article/abs/pii/S10577... for more detail. It certainly wouldn't be the first time the most profitable marketing strategy is unrelated to aligning with what's optimal for the consumer.
Translating the confusing science speak, basically: Appearance self-esteem takes a hit when they don't fit in a size. They take it out on the clothes: "I hate their stuff, they suck." They buy more of other stuff to compensate for the hit, whether non-sized accessories (I am pretty) or book/tech (I am smart even if I don't fit). People confident in their appearance are immune to the effect, and simply think it's size…
Having read the article, I can't venture a guess without feeling condescending...non-conforming? Compensatory?
Legitimately confused.