I don't recall Resverlogix ever explicitly stating the hypothesized mechanism by which apabetalone may synergize with rosuvastatin. I kind of recall them mentioning that it may have to do with effects of rosuvastatin on ABCA1 and reverse cholesterol transport. Here are a couple of references on that topic. Note, both are from the same investigator and the studies are done in mice.

Abstract 11181: Effects of Rosuvastatin and Atorvastatin on Macrophage Reverse Cholesterol Transport in Vivo

Rosuvastatin Activates ATP-Binding Cassette Transporter A1–Dependent Efflux Ex Vivo and Promotes Reverse Cholesterol Transport in Macrophage Cells in Mice Fed a High-Fat Diet