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.
Oh really? I always assumed as I never thought horizontally centring was ever a huge problem. Vertical was the actual hard thing for a long time (which is something table cells could easily do since the beginning).
You caught me in that I didn't read the full article, but I read enough to see that it has this CSS which centres on both axes: