You're right that they're not exactly the same, but they are intrinsically linked. Racism is an advanced form where, instead of just and us-vs-them mentality is used to protect against invaders, it is weaponized as tool to use lower echelons on the class hierarchy against themselves to empower the upper class and keep the masses from realizing how shit they have it.
Tribalism is the root instinct that drives it, its just been modified for use in class warfare.