Worth reflecting that the Bronx was a prime mover in Bison propagation in the early days. American Bison are not the only Bovid species with an element of hybridity….
I'm a big fan of the bison, so I'm well aware of how important the Bronx Zoo has been for the species. The bison might've gone extinct if the Bronx hadn't been an early propagator of them; Certainly many populations of bison wouldn't exist today as we know them without the zoos support.
The bison not being "purebred" is of no concern to myself, my avatar is a Przewalski's Horse - Another species that was saved by zoos and private individuals, but was hybridized in the process. (Although the P-Horse and now even the bison have been proven to have historically crossbred with their close relatives, even prior to being taken into captivity.)
The bison turning out to be "impure" on a species wide basis is just a
huge deal in the bison world. For literally the last twenty years, bison conservation has been driven by "purity". Supposedly "pure" herds were favored for greater propagation and reintroduction into new areas over "impure" herds. Nearly half a million bison exist in the modern day, but before this study came out, in the eyes of conservationists, only the twenty odd thousand supposed "pure" bison mattered.
No more, now every herd is on an even playing field. A better thing for the species on a whole, since "pure" herds have genetics that "impure" herds lack and vice versa. Since every bison alive carries a tiny bit of cattle DNA in their blood, that means that bison can now be exchanged between herds freely. Allowing for greater and better genetic diversity, which is always key to a healthy species and assuring it's continued existence.