What they're getting at is that the vertical aspect of margin:auto (that is, margin: 0 auto vs margin: auto) has absolutely no effect in your example. For your own credibility (since this purports to be a "complete" guide), this needs to be fixed and you need to add the table/table-cell method.

I think it would be best to simply replace that example with what is essentially the last example, which is much more useful. This codepen[1] shows the markup within another div. This is by far my favorite centering method, although it requires a fixed or percentage height while table/table-cell does not. It also avoids some of the bizarre quirks of table/table-cell, so there are times when one or the other is the best method.

[1] http://codepen.io/anon/pen/whHIL