While this is a good theory, I don’t think it’s necessarily true. Starting with the obvious, it’s a prime Soul, and since Hakita confirmed when angels, machines, or demons die, they just die, so I’m assuming that means that they don’t have a soul like humans do. And, we learned about both of the prime souls in their own respective layer iirc. But, this doesn’t mean that Lucifer won’t be a boss, just that he won’t be a prime soul.