I'm sceptic on the claim that this would evolve as a trait to assist tree climbing. Frankly, this sounds as silly for me as claiming that the Romans in Iberia developed some mutation to handle the local lions. The Vadoma are Bantu, and the Bantu reached the region roughly at the same time as the Romans were hitting Iberia.

And one might say "well, the mutation originated in a pre-Bantu people" - then why isn't it more widespread?

Instead I think that a simpler and better explanation is founder effect. Small population, avoiding marrying outsiders = higher representation of any mutant allele that might appear = new traits appear in a higher proportion of the population.