Although I agree that culture, language, religion can be passed down through the mother, the Somali identity is inextricably linked with the clan structure which is also the Islamic model. Allah says: Call them by their fathers (Surah Ahzab).
So in that sense, they can be Somali only somewhat culturally, and only through a 0.5 type of cultural assimilation. But they will have no clan because that is passed from the father alone and they will always be categorised separately for better or for worse.
Also the habar and Bah relations between clans is designed to denote a maternal relation and has nothing to do with clan lineages, it only adopts the name identification but is still a paternal clan lineage.
That's the official Somali line, but in reality, plenty of people got absorbed, whose only link to Somalinimo, is through their maternal lineage. And they got adopted by clans, who like to boost their population. For instance, the Boon/Aweer people in South Somalia, and this was during Colonialism, so they've only been 'Somali' for 100+ years.