Bear in mind not much is really known about Nsibidi, it's mainly the private domain of a secret society at this point. If they'd tried to use actual Nsibidi they probably get it badly wrong, which could be highly insulting or even seen as blasphemous as there are religious connotations, or get accused of cultural appropriation or both. I think creating something clearly separate but inspired by it and a homage to it strikes a pragmatic balance.