As others suggested, I think what they're prohibiting is listing keywords in the description.
It would lead to some rather strange descriptions if you couldn't use keywords naturally in the text. For example, if you have "vampires" as a keyword, how woud you describe your book to people that want to read about vampires without mentioning the keyword in the description?
"When John wakes up to find he's been turned into an undead, bloodsucking creature of the night, he must quickly adapt to his new nocturnal life while avoiding his newfound stalker, a legendary hunter of undead, bloodsucking creatures of the night."
And then readers would be like, ugh, why can't this author just call them vampires? I bet the writing stinks . . . Next!