There has been a fair amount of interbreeding with cattle, which means that the current genetic diversity is not as bottlenecked as you would expect, but also m

There has been a fair amount of interbreeding with cattle, which means that the current genetic diversity is not as bottlenecked as you would expect, but also means that most existing bison are not "pure bison". While the population was thought to drop to 100, there were several other herds discovered, the lowest the population ever got was probably closer to 1000 than 100.