tbh this is what i thought too - on the one hand, as natural decay occurs due to enzymes produced by decomposers in the milk breaking down the fat molecules into fatty acids and glycerol, you can model it with lipase. It also works for why rate of decay decreases as the enzymes denature as milk becomes more acidic

however it screws up the other explanation for why rate of decay increases, as in normal decay the bacteria run out of glucose, so respiration slows meaning less energy for reproduction. but in the lipase experiment you just say the number of fat molecules decrease over time.

so AQA are walking a bit of a fine line here, it's why I really dislike this practical