John 21:25 speaks in a general sense, not to anything specific in the context.
He quotes from two different books which were not talking about the same subjects and truncated them to make them sound like they were. If you quote the two passages in their entirety, I think it is pretty obvious they can not be put together like they were.
First he quoted parts of Matthew 28:20
19Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
20Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the world. Amen.
Then he quotes parts of John 21:25
24This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true.
25And there are also many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written. Amen.