Not exactly into the theory myself, but just to explain it, the arguments in favor of the "Orion is the King and not the Eldari" theory are that:
-Caly's recollection of events is sus due to being trapped in a box that Linus presumably modeled his implant scramblers on (what makes it extra sus is that the implant scramblers used to have a different model in 0.4 until Cari changed it in 0.5 to resemble Caly's prison box, meaning there could be something more to Caly's prison than just being a mere box)
-Since Caly's recollections are sus, it's possible that the Anima story is a fabrication or there is more to the story. The theory goes with the latter, arguing that at some point the Eldari started fearing death, and so devised a means to trap Anima and Caly's realm in a state of immortality. This method also resulted in the Gates of Acheron closing and the dead not being able to move on into the afterlife
-Anima, now Idriel, decides that the only way to defeat her corrupted lover is by reviving the one person that could potentially defeat him: the old king. This comes with the caveat that even as a new incarnation, he still retains his arrogant streak - something that Orion has been slowly exhibiting as time goes on. But since he's the only one capable of defeating the Eldari, he will have to undergo a humbling process that would ultimately make him go against his nature, going from former villain to hero instead. This also goes with Idriel telling Orion to "choose tails" and "break the cycle," as it is in reference to him going against his former nature to become someone better.
