However, the study is complicated since rats are sensitive to abdominal gavages and it tends to reduce their lifespan. Similarly, there are studies that show chronic administration of olive oil increases life span.

So you have a situation where one control (water only) leads to decreased lifespan and another control (vehicle - olive oil only) tends to increase lifespan. Neither of these are ideal controls since they are known to affect the rat.

That said, they did fancy stats and presumably accounted for this fact.