When temperatures drop below -10 with FZFG you get into some very bad icing conditions, both for the airframe/wings and the engines. I have no idea if they deiced before departure, or if they did, what kind of fluid they used.
Type 1 (Clariant Safewing) will give you less than 10 minutes HOT. Type 2 nearly one hour.
If this takeoff was done without a proper engine run up before releasing the brakes... I’ve seen engine blades pick up 2-3 cm ice from landing to parking.
Wing ice can kill you. Engine ice can kill you. Combine the two and you need to be very careful and get everything right or you will get into trouble.