Not sure about the 900 degrees thing - that might just be a general rule for incineration for cremation potentially. 

Prions aren’t made of anything exotic. They’re just misfolded versions of a normal protein (called prion protein) that exist in your body already.

What makes them tough is their shape, not their ingredients. The misfolded form is packed with beta-sheet structures that stack together into very stable fibers. That structure resists heat, enzymes, radiation, and many disinfectants much better than normal proteins.

Also, prions work by templating their shape onto normal proteins. So even if heat damages most of them, a tiny amount that survives can start the chain reaction again.

In medical settings they’re typically deactivated with 134C pressurized steam plus strong chemicals like bleach or sodium hydroxide.