It's limited by the FAA to a maximum of 100Wh. The Pinebook Pro has a battery of 10000mAh at 3.8V (I think), so 38Wh. There is plenty of room for a bigger battery.
But that is 100 watt hours. If my math is correct (which it is possibly very wrong) this battery is only 50WH. My guess is this is to help keep cost down not because it wouldn't be allowed on a plane.
Does this really limit the market for extra batteries a lot?