Somali is just a social construct. It is not defined by phenotype or genetics. Somali Bantus are Somali.
I thought you were some miskeen faarax hating xalimo feminist, but little did I know you were of the Margaret Sanger persuasion. Abaay, abaay.
Somali ethnic identity like Ethnicity/Race in general, might be a social construct but it's one constructed around real shared realities like shared ancestry(which is backed by genetic studies), history, language, physical-ethnic traits and culture. Anyone cannot be Somali, it's the dominant majority that decide who and what is or isn't Somali. They define exclusion and inclusion areas, and boundaries that entails who is Somali.
I can go out there an say i am Han Chinese but that wont make Han Chinese people accept and embrace that. I could argue and troll my tail off about it and it would change things in actuality. It's the chinese that control the definition.
The purpose of a definition in sociological terms is to discuss groups with historical, social, mental, physiological, linguistic, cultural, similarities.
We have no hope of understanding anything if two polarized groups are collapsed into the same name and this is the case with Bantus who have a seperate history , origin and even social experience than Somalis, as well as being physiologically different. It would be a crime and erasure against them to that.
Personaly i think naming the country after an ethnicity creates problems and confusions. A problem that wouldn't exist in say Ethiopia, i can be Ethiopian but that wont make me Tigrayan.