The New Age business deliberately distorts history. The Celts are everything from the survivors of Atlantis to brothers of the Hindus, depending on the spiritual needs of the clientele. It's designed to sell books, CDs and other items. The Celts of their marketing are seldom the Celts of history.
As for Ellis ... meh. I reviewed one of his books for this site. I think he is a modern polemicist with an agenda to turn the Celts into Europe's great Uber-race. He's at the opposite end of the New Age - the nationalist.