This study looked at the blunting of the blood glucose spike that follows sugar ingestion. They determined that protein was three times more effective than fat at blunting the glucose spike. The abstract does’nt mention the type of fat used, but that would also make a huge difference in the results.
Protein is a known stimulator of insulin, especially when consumed with carbohydrates. It is almost certain that the sugar/protein group had higher plasma insulin levels than the sugar/fat group. Higher insulin levels would drive the blood glucose into tissues faster than lower insulin levels.