I was discussing this with someone some yrars back. The origin of the Igbo word seem obvious. Ndi Owerri means Owerri people, Ndi Enugu means Enugu people, Ndi Yoruba means Yoruba people. So it is not hard to see that Ndi Gbo meant ALL people originally. Ndi as used by Igbos is thesame idi from idile used by Yorubas. We are descendants of thesame ancestors just a few thousands years back. Idile means family in Yoruba.

Gbo in Ndigbo is thesame as Gbo or Gbogbo in Yoruba which means all. It occurs in the title Akarigbo of Remo which means Akarigbogbo Remo. The akari or head of Remo land. Akari means same as Olori from ori meaning head.

So when Igbos say Gbo kwe unu! They are saying Gbogbo hin e ke or Gbo hin e ke which is Yoruba for all of you shout.

NdiGbo means everybody in reality. That is my view.

When ndi onitsha meets ndi owerri and etc, they refer to evetybody as ndigbo meaning everybody
Same way onuigbo means mouthpiece of all