It was only slightly more nuanced than that, but I do agree with the author's premise that lack of standardization makes buying clothes that fit well difficult.

It was when they started mentioning the statistics of how many adult women can't wear certain sizes/brands that I realized the heart of this complaint was that too many American women are overweight.

There's two things at play here. People who don't give a shit about how their bodies look are less likely to be vain enough to buy designer clothes. Obese people are hard to make clothes for due to how we all store fat differently.

At the the of the day, if it were profitable, companies would adapt to the size of their customers. It's not profitable (enough) so we get vanity sizes as a compromise.