It's kind of a well-known phenomenon in commercial food safety. It's the acid that does most of the work, but the minimal water activity also helps. Coating food with acidulated oil helps prevent the growth of many microbes that require water and more neutral pHs to thrive. Here's a link to the "ur" study. And the "nut" of the abstract:
Similar studies replicate the finding in various foodstuffs.
Mayonnaise retarded but did not prevent the growth of S. aureus or S. typhimurium in salads stored at 22 or 32 C for 24 h. Increasing the concentration of mayonnaise in salads increased the degree to which growth of these organisms was delayed. Contrary to popular belief, the presence of mayonnaise in meat salads tends to retard rather than enhance growth of food-borne pathogens. However, addition of mayonnaise should not be considered a substitute for refrigeration for preserving meat salads from the growth of food-borne pathogens.
Similar studies replicate the finding in various foodstuffs.