Diabolos isn't a Primal in FF14. He's a Voidsent. Which is actually kind of like a Dominant/Eikon from FF16, or a Lucavi from FFTactics.

In the Spirit's Within movie, Ross has dreams of an alien world. And they describe the meteor as a kind of "noah's ark" full of ghosts. Which makes her phantom a kind of Diabolos, in parallel structure.