Given that the 13" has got a 58 Wh battery, it means that the stated 10 hours of battery life are calculated based on an average consumption of less than 6 Watts for the entire computer, while we know that the CPU alone is designed to a ~15 W spec when in active use, let alone the other components in the laptop.
Supporting this, according to a recent Accidental Tech Podcast (which admittedly gives us exactly one datapoint), realistically any kind of non-trivial work on a 2018 MBP will send it down to 5-7 hours of actual battery life. That said, 10 hours is probably true for when the computer does something less straining or something it's highly optimized for.