Maybe I have misconstrued something, but I didn't see that Tjampel was arguing the subject matter is irrelevant. I understood him as saying the Buddha's presentation was appropriate to the audience of his time. If he were teaching now, he might have conveyed it differently. A metaphor is a way of presenting content. It seems to me that by saying the six realms are metaphorical, you are agreeing with Tjampel that the issue here is about presentation.