There were already many people claiming the be the legit somalia in the 90sThey don't need to control the rest of Somalia, they just need to claim to be the only legitimate Somalia, and keep the rest in chaos. Literally speaking they have been screwing themselves over trying to be "special" when all they had to do was say "We are somalia" and prevent any strong entity from forming in the South, which would be easy with the new funds they get by being a real country.
FGS is considered a legitimate government(by the UN) despite not controlling 90% of Somalia's territory.