The Primarchs discussed it for a long time before ultimately abandoning any attempt to expand upon the theory of nine angels and nine demons. Repeatedly reinforcing that theory would only lead humanity further down the path of irrationality, and none of them wished to be worshipped as gods by mortals.
As a compromise, the Primarchs prepared to explain the return of the traitor Primarchs as a task entrusted to them by The Emperor.
"The Emperor gave me a difficult task: to find each of His children who betrayed Him and bring them back, so that they may kneel before the Golden Throne and repent. Only The Emperor can judge their crimes, and only The Emperor should."
Jaghatai answered Stunus without mentioning Lin Fan. After all, to the countless Astra Militarum regiments, Battle Sisters, Space Marines, and other organizations across the vast Imperium, the name Lin Fan was both unfamiliar and ordinary. Even if they used the alias Lin Fan had employed during his early operations in the Imperium—Holy Soap—it occupied no more than an obscure place in the vast archives of the Departmento Munitorum and the Inquisition.
Besides, who would believe what Lin Fan had done if it were told? His case was utterly unsuitable for propaganda aimed at the people of the Imperium. Some Imperial citizens did not even possess a concept of gravity; they regarded the effects different planetary gravities had on their bodies as a supernatural curse.
If they truly had to explain Lin Fan's origins, it would not be impossible. They could simply toss out the words "chosen by The Emperor," then sternly warn anyone asking questions not to pry. Clever people would know what to do.
"Bring all of The Emperor's children back... then judge them..." Stunus lowered his head toward Jaghatai in thanks for