Somali is an ethnoreligion. Our founding, unlike Quraysh, was built on the premise of our ethnicity and our Deen (the evidence of that is the history, regardless of its veracity, of our Qabil - how it is intimately tied to Al-Islam, which was supposed to emphasise our intimate ties to the religion). Without one you aren't Somali. Without the ethnicity, you are a Muslim (May Allah bless you and you are still my brother in Islam) and without the Deen you are no different than Oromo (the facade of Somali and may speak Somali but still isn't Somali - you leaving is the demarcation) You leaving Islam means you are Gaal (quite literally no different from an Oromo).
Think of it like this, why do most not tell their parents, due to fear of being ostracised from the community right. Well, imagine their whole abtiris, no one will claim this child, it is essentially a bastard child - you are not claimed by any Somali within your own family let alone the community, hence you aren't Somali.