What was the purpose of the apostles? To teach scriptures? Write Scriptures? To hand down the Lords messages? Could only those messages have been in writing form?
The only teaching that we can be sure has been passed down correctly is the teaching that’s in the NT because there are manuscripts to which we can go back and refer to.
Anything else, ESPECIALLY something that was added or *discovered* years (decades, centuries) after the fact, is highly suspect.
There is simply NO WAY to verify that it had been handed down by word of mouth or tradition without error.