That’s 19 min from a cold body with no foreplay, you take a pic or two and fiddle in the menus that time begins to drop. I got to use an r5 today out at a Motocross event and well it turned off from only doing photos. This was using the ef to rf adapter and a 200mm f2. This was used for over a time period of 2.5-3 hrs and it was 86 degrees out. I’m glad that I didn’t buy it and needless to say my friend who did is pretty disappointed. I love 1d bodiesR5: no overheating in 4K@60p (external recording) looks pretty damn good to me. 19 mins 8K in one take also seems not tooo bad to me.
I wonder if the camera lay flat on a table with no convection from underneath or if it was mounted "free" on a tripod. Also, Canon recommends an external fan during recording, that might also greatly improve cooling.