He does mention at one point that sometime soon it won't be necessary:

> Note: This step will become much easier and concise when the sibling-index() and sibling-count() functions gain support (and they’re really neat). I’m hardcoding the indexes with inline CSS variables in the meantime.

The inline links there go to https://css-tricks.com/almanac/functions/s/sibling-index/, which is pretty nifty honestly.