Caesar was brutal in Gaul, a mere part of the vast Celtic-speaking world, and his campaign probably impacted a decent amount of Proto-Basque or Para-Basque speakers as well. Many historians and linguists prefer to see “Celtic” as a linguistic distinction only and therefore are reluctant to speak of “the Celtic people”. If you ever see anyone online write of “Celtic genocide”, you can be pretty sure that the person has little formal training in this field and is going off of low-quality pop-sci publications or pure internet amateurism.