Not to mention that he simply could not pass the first test, even if he did, accepting the Offering God Demons' Divine Soul Baptism would likely expose him.
After all, Thousand Changes and Ten Thousand Transmutations could change his appearance but not his Divine Soul. If the Offering God Demons were truly of the Emperor-level, there would be no way they wouldn't see through him.
This could be inferred from his foreknowledge.
His foreknowledge showed that the Offering God Demons would ultimately kill him in anger, possibly because they discovered his deception.
Moreover, one's Divine Soul is the most important thing; how could one let another existence's Divine Soul enter it casually?
Even if the Offering God Demons were not able to detect his true identity, Su Nan couldn't readily allow them to enter his Divine Soul.
"There's no other way, just take it one step at a time and finish the first part of the Task first."