Yes, I agree. I felt certain it must be Apocynaceae when comparing it with a Landolphia fruit too. But since Natal Plum is a thorny shrub, I wonder about the form of the plant. I suppose a thorny shrub is what you find in a dry place with animal browsers. It would be smarter to be a vine in the tall forests of Kalimantan. I will go with the vine guess too. I wonder when if ever, we will know the answer....