With that said I sort of disagree with you. Prions are particularly attractive to a hypothetical bioterrorist due to the difficulty in detecting them, their durability and also the delay on effects. As I said above, once you know you have a problem it will be too late.
Keep in mind these are just the prions that nature has produced. It is quite possible that humans are able to modify existing threatening prions or develop whole new classes of threatening prions that haven't developed in nature.
Overall I think the most effective strike from a bioterrorist won't come from a single attack vector but a well timed combination of vectors. We've seen how something as minor as COVID has taken all of our attention and resources, now imagine if a terrorist timed something like COVID with a follow up strike like prions.
Terrorism is about fear, and that comes with spectacular actions even if it kills relatively few people, not with stealthy poisoning of big population.
You're not gonna have anything remotely as impactful as 9/11 with prions, even if you killed a million people in the long run.
Is it a good weapon for a nation state going at war, or committing a genocide. Definitely yes! But it sucks for terrorism.
While the kinds of people who would fly planes into buildings, release nerve toxins in subway stations or club entire villages of outsiders to death have always existed the ability to research and manufacture prions did not exist in 2000BC.
That combination of desire to destroy the world and the legitimate ability to do it with rapidly more accessible biotechnology is a disconcerting reality that we need to account for when discussing theses things.
Imagine a religious extremists organization mass producing prions and placing them in key parts of food supply chain. We probably wouldn't have a clue until it was too late.