If the initial seed was produced by self pollination but the population of Hoya was genetically diverse then the seed could have been produced by a natural hybrid.
As long as a Hoya plant has not produced true to type seeds we can't say for sure if it is a species or an hybrid.
Fortunately Hoya in the trade are propagated by cuttings, not by sexual reproduction so preserving the integrity of a Hoya breed ( species or hybrid ) is not an issue : there is no reason why a grower would mix true to type seeds with hybrids as they don't use seeds for the propagation of a breed.
However seeds are very useful : whether they result from cross pollination or from self pollinated hybrids, originally all breeds are hybrids.
Herbs and Spices, Hoya, Hoya seeds, Orchid seeds, Ferns, Fern spores are for sale on aleyagarden.com Plant care is available in various posts of this blog. Contact : firstname.lastname@example.org