That is not so.
"I do this so that you will know the
full truth about everything which you have been taught."
is a statement about the truth taught to Theophilus; so, unless you want to contend that saint Luke sees Theophilus as an exhaustive cyclopaedia of Christian truth then it follows that saint Luke aimed to write down what he knew Theophilus had been taught. That implies a limit that your claim denies.
Well, what I underlined is this:
I do this so that you will know the full truth about everything which you have been taught.
That is not obvious, it is not even implied.
Nope, saint Luke writes that he is reiterating the full truth about what Theophilus was taught.
Yep, it is implying that saint Luke or some other person will teach more to Theophilus later. We need not look too far to see the "more". It is present in the book called "The Acts of the Apostles"
Dear Theophilus: In my first book I wrote about all the things that Jesus did and taught from the time he began his work until the day he was taken up to heaven. Before he was taken up, he gave instructions by the power of the Holy Spirit to the men he had chosen as his apostles.
Acts 1:1-2 GNB