This region is held as one because its religious history is a single history: West African traditions carried across the Atlantic by enslaved people and rebuilt under conditions designed to prevent exactly that.
What was rebuilt was not a copy. People from many peoples and languages were thrown together, and what emerged in Cuba, Brazil and Haiti drew on several West and Central African sources at once, took cover under Catholic saints where it had to, and developed its own priesthoods, liturgies and lineages. These are not survivals. They are religions made in the Americas by people using what they had.
The written record is correspondingly awkward. For the African side, the earliest substantial descriptions are by traders, missionaries and colonial officials. For the American side, much of the archive is legal and ecclesiastical, generated because the practice was being prosecuted or policed, and Brazilian and Cuban authorities kept files on it well into the twentieth century.
A further complication is the category itself. The consolidation of a single Yoruba identity across peoples who had not previously used one name owes much to the nineteenth century, to mission schools, to standardised written Yoruba and to returnees from Brazil and Sierra Leone. The religion now studied as Yoruba tradition took shape in dialogue with the diaspora rather than simply preceding it.
Both sides of the ocean are living traditions with living authorities, and the atlas describes rather than instructs.