If you didn’t read the rest of the paragraph, you should. It was comparing against variants of polygamy, plus 2 cultures that had polyandry, which I discussed elsewhere. Western-style polyamory didn’t even make the rankings. I can only think of one other culture (the Mosuo) that might count.
Like I said, it might be an abuse of terminology to call this all monogamy, but natural language is inherently imprecise and this isn’t an academic audience that can digest heavy jargon.