One thing that strikes me is that we now have three species (if we assume this is a new species) in the sigmatopetalum group from Sulawesi that have white or whitish staminodes. Possible endemic pollinator influence? The only other sigmatopetalum from Sulawesi that I can think of atm is P. celebesense which does not.
If we follow the natural hybrid idea then my guess would be P. sangii var ayubianum x P.celebesense. The celebesense could have contributed the yellow margins to the petals, red/purple speckling under the leaves, smoothed out the sangii pouch and influenced the shape of the staminode. The dorsal shape is all sangii to me. I have no problem with the idea that natural hybridisation can produce new species so long as there is a breeding population that is surviving and has been subjected to natural selection for a sufficient number of generations to achieve some degree of genetic stability and coherence as a population.
Hmm ... I hope that flower produced a pod and that seedlings are on the way !