That is actually not supported by the scholarship. The idea comes from the Roman record – and Romans had cause to malign Celts in their propaganda masked as scholarship. And, while Romans didn't call what they did "human sacrifice," they employed ritual murder with probably no less enthusiasm than the peoples they conquered and then accused of the "barbaric" practice. It was a matter of semanitics to Romans.

Here is a short brief on it: "The Roman abhorrence of human sacrifice presented by ancient literary sources stands in contrast to the frequency of rites requiring the death of a human being performed by the Romans during the Republic (509-44 BCE). After examining the ways our sources talk about ritual murder, especially as it was practiced by foreign peoples and subversive or tyrannical elements within Roman society, this discussion turns to the issue of the forms of ritual murder performed by the Romans. Of these various rites, the only one clearly identified by them as human sacrifice, that is, as an offering to the gods of a human life, is the live interment of Gauls and Greeks. Other forms of ritual murder-the burial of unchaste Vestal Virgins and the drowning of hermaphroditic children-were not, in Roman opinion, sacrifice. This distinction made the disposal of Vestal Virgins and hermaphrodites acceptable." [PubMed]