Isn't the real mis-step of this study that they put the rats on a HFD 3 (three!) days before testing? I thought it was common knowledge that the transitory periods of ketosis elevate blood lipids until your body adapts to its new food source / arrives at a new equilibrium. I would think their conclusion should be

"Taken together, the present data indicate that (!__consumption of__) -> <__transition onto__> a HFD prior to laparotomy prolongs the neuroinflammatory response to surgery and induces persistent memory deficits in young adult and aged rats. We demonstrated that both age groups who ate HFD for 3 days prior to surgery displayed robust cued-fear memory deficits, and aged rats additionally exhibited contextual memory deficits. These memory impairments were associated with exaggerated and prolonged (3 weeks post-surgery)"

Which is just another way to say, don't inflame your entire body with a new diet prior to undergoing surgery (which in and of itself is massively stressful).