margin:0 auto is the (probably incorrect) incantation I have always reached for
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.
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:
body {
display: grid;
min-height: 100dvh;
place-items: center;
}