No lyin'. I have an iPad Pro 12.9", 5th generation. I download content on Netflix, Prime, and to VLC prior to flights. During flights I have the iPad in airplane mode and use my AirPod Pros for audio. On my flights from California to Texas, ~3 hrs, I use somewhere between 5-8% battery life. Brightness is probably on the low side because the cabin is dim.
Airplane mode (turning off most of the wireless) seems to be a big factor as well.
That sounds wild. Apple says iPad is rated for up to 10 hours of video playback [1]. The display alone should consume more than that.
1: https://www.apple.com/ipad-pro/specs/