Let me give you an analogy. Lets take Stephen King, a well known author. He begins writing a book, telling a story. Now, in his telling of this story, or when hes finished with it, he doesnt say that he is restricting the information or narration of this story to what is contained within that story only.
Anybody who reads King knows that he often writes sequels or interchanges characters in different books. So, when we finish one book, we dont know theres going to be a sequel until the sequel is published. Sometimes, he is finished with a particular story and does not write a sequel, but hes not finished with a particular character. So, that character may show up in a different book. For example, there is a character, named Cynthia I believe, who is in the books Rose Madder and Desperation. So, while he was finished telling the story of Rose Madder, he wasnt exactly through telling Cynthias story. How do you and I know when he hasnt said all there is to say regarding one particular book or character? Well, when the sequel comes out or we find a particular character in more than one story.
So, when he writes a book, does King specify whether or not that book is finished? No. Now, as Ive shown, its easy to know when he wasnt finished, a sequel comes out. But, if he doesnt tell us there is going to be a sequel, is that solid evidence that there will or wont be? No, but its reason enough not to dwell on it until it is done. If he doesnt say that this story is finished not to look for a sequel, is that solid evidence to believe that there will not be? Well, no. But, its also no reason to presume there will be, either.
Its the same with the Bible. Just because Paul didnt restrict what he wrote to particular epistles or Scripture, does not by necessity lead to the conclusion that there is more to the Gospel. Of course, we know Paul did not restrict his truth to one letter to one church. How? Because there are numerous epistles of his in the NT.And, theres absolutely no evidence, biblical or otherwise, showing that he possessed any knowledge of any truth not contained therein.
Its not just this letter that serves as an example of the apostles preferred method of teaching when they were unable to instruct in person.