Confused about where 'thin/fat' fit into the Roya Order of adjectives

I know that you're meant to order adjectives based on their type (quantity, opinion, size... etc.), but what type do 'fat' and 'thin' fall into? Are they size or shape? I feel as though I can just as well say "a fat old woman" as "an old fat woman".

A related question: when researching the topic, I found this confusing blog (https://www.grammarly.com/blog/adjective-order/), in which the author says:

'If you’re stuck on whether to use a comma or not, here’s an easy trick: if you can add the word “and” between the adjectives and reverse the adjectives’ order without losing the sentence’s clarity, you don’t need a comma.'

This says you don't use commas between coordinate adjectives, contradicting what most other online sources and many style guides say. The writer immediately contradicts themselves in the next section by writing, 'It was a loud, crowded concert.' Am I missing something, or is the blog incorrect?