I'd read that many times already but I took your word for it that it was in there. The crucial miswording is in step five, "Style: Choose the style to display in your header or footer." I didn't understand, and I think many others don't understand, this is not the style of the text you are putting in the header. This is the style of the text that will be replicated in the header. For my purposes I made a Paragraph Style of STORY AUTHOR. Then I used that in the body and then pointed to STORY AUTHOR in the "Style" field of the Running Header Text Variable. Thank you for making me re-read it.
I imagine if someone needed a field that was not in the body text, they could create a hidden bit of text.