Reading a bit further, I found that about 10 years ago Rollinias were reclassified as Annona. So the plant in question is listed here along with its synonyms:
Annona mucosa information from NPGS/GRIN
I don't have information on Annonacin levels in the fruit. But I did find this interesting article on a simplified method for checking directly from fruit pulp:
http://www.mdpi.com/1420-3049/14/12/5235/pdf
The Ag lab at the HILO GRIN repository would probably do this for the usual fee, or perhaps you have access to other resources.
Once the concentration is known, it can be compared to the published conc. of soursop.