That doesn't make sense. Wild bettas tend to be less aggressive and more hardy than the completely inbred splendens.
Also Betta raja are from the same complex as pugnax so essentially the same breeding methods. Ergo, they would also be very unlikely candidates to cross with splendens.
So how is it possible for a mouthbrooder and a bubblenester to spawn? This is the crux of the issue. With mouthbrooders the female spits the eggs into the male's mouth and he then proceeds to carry them for a couple of weeks until they hatch. Therefore, if you used a male splendens he would be trying to put the eggs up in a nest, and if you had a male mouthbrooder, he would be trying to carry the eggs around in his mouth.
I think the confusion comes from the translation of 'raja' as king. So people are assuming that Betta raja are or played some part in the creation of giant and king bettas.
Here is a thread that Indjo posted about the creation of giant bettas