@phoenix It looks to be statistical inference based on sampled and modelled data. On p. 57 of the report: "To model customer use, we measure the power consumed by a product while it is running in a simulated scenario. Daily usage patterns are specific to each product and are a mixture of actual and modeled customer use data."
The number of devices for the statistical inferences is: "In fiscal year 2018, we sold 217,722,000 iPhone devices, 43,535,000 iPad units, and 18,209,000 Mac products."