Prions are quite stable, and also they don't need to stay in the soil for long, just enough to get reconsumed. Supposedly that's how CWD (chronic wasting disease, not coarse woody debris), is spread among deer.

Edit: in context with composting, overall temps would be higher in such a pit but not by much. Its anywhere from room temp to 140F/60C. Prion destruction is a lot higher temp wise. As for bacteria in the pile, maybe? It might be more likely to become meaningfully degraded in a compost pile instead of normal soil.

As for cwd prion bio accumulation, it's been hypothesized but not demonstrated (like grass picking it up from the soil itself). It's spread in saliva and indirectly from the environment which is probably why you shouldn't feed deer in areas with cwd and explains a lot of the spread. Also apparently the scrapie prion can endure for 16 years. Wtf.