Given that the only battery life number they quote is for video playback (with no specs about the video format, bitrate, resolution, full-screen state, etc), I

Given that the only battery life number they quote is for video playback (with no specs about the video format, bitrate, resolution, full-screen state, etc), I suspect that number just happens to be extra high when you choose the optimal video in the optimal format and play it full screen (so the OS can stop spending cycles on UI rendering). Note that Apple specifies 10 hours of "iTunes Movie playback" for the 15-inch Macbook Pro. It's probably possible to improve on that with some other format. Apple also gives a quote for "wireless web" usage (the same 10 hours). The 15-inch Surface Book 2 is also significantly thicker and about 5% heavier than a 15-inch Macbook Pro. So I'm sure more battery plays a role as well.