I disagree that's what people mean by "Centering a div" and the hilariously repetitive corpus of programmerhumor jokes on the matter seem to disagree too.

And in either case, I don't think the article fully agrees either, since the thesis of it relates to a strictly horizontal element. If the article doesn't intend to disagree, then it was just a framing device they used in the writing that has some distractions regarding the vertical component unfortunately attached.