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.

a quick look at wikipedia will show you are wrong

"In 2015, researchers at The University of Texas Health Science Center at Houston found that plants can be a vector for prions. When researchers fed hamsters grass that grew on ground where a deer that died with chronic wasting disease (CWD) was buried, the hamsters became ill with CWD, suggesting that prions can bind to plants, which then take them up into the leaf and stem structure, where they can be eaten by herbivores, thus completing the cycle. It is thus possible that there is a progressively accumulating number of prions in the environment."

Proteins are also typically denatured by heat, and yet cooking does not remove prions. Prions are hard to get rid of.