Maybe I misinterpreted what Feral was getting at, but at least from my reading of it, he’s saying the myth was ‘half-true’ in the sense that that propaganda was effectively sold to the lowly enlisted men to give them self-justification to fight. Whereas the truth was indeed that the whole thing was just for slavery. The myth then became even more useful after defeat to help them accept it ‘honorably’. His conclusion about what people believe and feel ultimately having a larger impact than the actual truth seems to confirm that reading, but I could be wrong.