Specifically, the researchers found 5 Yamnaya skeletons well-dated from 3000-2,500 BCE which display characteristics of "horsemanship syndrome" - ie, stresses and changes to the pelvis, thigh bone, hip socket which are seen in confirmed horseriders.

They aren't saying this is definitive proof of horseback riding. There's some speculation riding another animal (like a mule) or using the same muscles in a non-riding activity (barrel making, basket weaving) could cause similar characteristics.

But since it's already established that the Yamnaya people kept domesticated horses for milk at this point, it's possible.