Edo and Yoruba people shared royalty. Onitsha which is associated Igbo people was actually settled by Benin. Lagos which is largely associated with Yoruba people was founded by the Army of Benin. So yes if you look at the map there is a lot of territorial overlap and interrelatedness among those Southern ethnic groups.
The ethnic groups in the west & east have no overlapping lands. The overlap is in the middle (the region classified as South South).
Yoruba & Benin share royalty via Oranmiyan & overlapping admixture with Lagos Island (a small part of Lagos - not the whole Lagos), Owo, Akoko & Ishekiri. Yoruba & Igbo don’t share any overlapping admixture cos there are buffer ethnic groups between the two. However, Igbo & Benin share overlapping admixture with Eshan, & a lot of groups in the Delta region.
I think the overlapping thing all over southern Nigerian is linguistics since almost all the groups are Niger Congo speakers save for a couple of groups in the eastern part. And what that means is that they’re all part of a larger group that broke apart thousands of years ago due to different migration patterns or perhaps they developed the similarities when they arrived at their current location.