I've more often seen collectivism as an opposite to individualism: that is, a desire or tendency to work together in groups or to focus on the welfare of a group more than that of individuals in it, or in some cases a focus on preventing harm over maximizing freedom. Tribalism is better because it doesn't have any other meaning. Also, the tendency Orwell is talking about probably evolved during conflicts among hunter-gatherer tribes, but I have no citation for that.