Without having read the article: the art market is as rigged as professional wrestling. The notional value of a sale might have little to do with what the buyer has to give up to achieve a sale. It is in the interests of acquirers of art pieces to acquire them at notionally inflated prices, since that imbues them with value in the eyes of philistines and so their expected resale price.

Postscript: Having read the article, the high-end British auctioneers like Sotheby's, who handled this sale, are perceived to engage in this kind of price-fixing less (i.e., they are supposed to take measures to ensure that participants do not "cheat" by having different parties collude), although there have been scandals.