My own paraphrase would read ...
Hold fast to the Day of the Lord teaching, whether you heard it from us in person when we were here ... or whether you have read it from this epistle.
Correct. The nonsense that the RCC has foisted on the people is sad indeed. To try to claim that tradition meant something that was not in print was a way the RCC could control not only the people but governments as well. And they will be punished for it also.