If Duan Ling Tian was an ordinary Void Initiation Stage martial artist, it would naturally be impossible that he would think so much.
The reason he thought so much and his guess was based on evidence was entirely because he’d fused with the two lifetimes worth of memories of the Rebirth Martial Emperor.
Moreover, he’d once encountered a formidable soul that wanted to destroy his soul and seize his body.
So, no one was clearer about souls seizing another’s body than Duan Ling Tian.
"According to the memories of the Rebirth Martial Emperor, if one’s soul is sufficiently strong, one can directly destroy the soul of another and occupy the person’s body, effectively replacing the person."
"However, if one’s soul had suffered an extremely great blow and only an incomplete soul remained, then it would be difficult to destroy the soul of another and occupy the person’s body."
