Everything is synthetic and made mostly of plastics especially in aircraft (aside from lightweight metals. An APU fire would yield thick black smoke from all the hydrocarbons.
The video you’re referring to resembles nothing close to what a fire burning in a plane would resemble. The volume of vapor/gas chemical and rate of discharge indicates a rapid release more than likely from a system designed to release said vapor as seen in video.
But you knew that already..
Try again