The legal argument is a smaller, but incredibly valid element of Somaliland's. However, there is clear precedent of several countries that have voluntarily entered into unions and afterwards reinstated their sovereignty, which is the primary case for SL.
I think that especially more recently, the markers for international recognition will become ethnic/religious self-determination.
However, as you said, if that was the bar then there would be hundreds of African nations. Hence, why I think there would be even more extreme standards placed on aspiring African nations.
They need to convince why they require a state more than hundreds of other groups. NB: when I say, require, I mean it as different to deserve, you can deserve something but not require it.