Dendarii
International Debutant
It's an average of five white players taken across the season, not a restriction for every match. In some of the ODIs against New Zealand they played six - De Kock, Du Plessis, De Villiers, Miller, Morris, and Pretorius.There's only space for five white players.
So it's not impossible for De Villiers to come into the test team as the sixth white player, even if they'd have to account for it in other matches. Although there would still be the option of maintaining the quota by him replacing Duminy and Parnell being the third seamer.