That's not a very well thought out article...incomplete at the least, but I follow its drift, and that's not what I did, because the procedure it describes results in an APFS High Sierra on an internal SSD while I wanted mine HFS+.
So, in order to get a HFS+ High Sierra on my internal SSD I first ran the installer on an external volume, where it installed as HFS+ by default, and then cloned that volume to an internal HFS+ volume.