If people supporting Imam Ali were supporting him for Arab tribalism then all of those supporters should have been from Banu Hashim.
What it actually proves is that people who did not accept Ali as Prophet’s heir did it due to Arab tribalism.
Yes, it proves the point that tribalism was only an excuse not to accept Ali as Prophet’s heir, because at Saqeefa the same concept of tribalism and kinship was used to justify Qureish’s right to khilafat.