Do Third-Party Guarantees Suppress Bidding?

This report is available to subscribers to AMMpro. Each subscription comes with the first month free. Feel free to subscribe to read the report and cancel before the month is up, if you’re not satisfied with the value of the content.
The New York Times ran a column on the art market over the holiday weekend that is a muddle of ideas about how guarantees affect prices. According to the column, Sotheby’s and Christie’s have diverged in their approach to guarantees. The basis for this judgment seems to be the results of the November sales.
Because Christie’s saw two of its highest-priced works sold without guarantees but after rounds of intense bidding—and the lead lots at Sotheby’s and Phillips had third-party guarantees but sold without much competition—the author concludes on the basis of this small sample that third-party guarantees suppress bidding while works without guarantees result in “competitive bidding.”
Here’s how the Times puts it and if someone can explain what a “‘true’ market price” is they get extra points):
But the auction houses’ reliance on third-party guarantees to source and sell big-ticket artworks, particularly at Phillips and Sotheby’s, does raise the question of how much that $25.6 million for the Richter represented a “true” market price. Values can be inflated when auction houses offer competitive guarantees to secure major consignments. These guarantee can then be transferred to a third party, who will either buy a work at a sale, or be rewarded for their unsuccessful bidding. More recently, some third-party guarantors — called “irrevocable bidders” at Sotheby’s — have also been paid fees if they are the purchaser. These confidential arrangements, denoted by symbols in the catalog, can have the effect of deterring bidding in the salesroom.