There is a change of tenses in the verse.  “What he has seen and heard” points to the existence of Jesus while “he testifies” refers to His mission.

The teaching of Jesus is not some great thoughts which He has derived from His study; He is teaching facts because He has actually seen and heard these things.  He is a witness to their validity.  There is no room for discussion; these are the facts and you can either accept or reject them.  Period. What has Jesus seen and heard?

“no-one” is not meant literally as can be seen in the following verse.  What would be the outcome of taking this literally? Why is this non-truth in the Bible?  It is an exaggeration for emphasis. Can you think of some other occurrences of a non-literal usage? One example is John 21:25.  How can you know if something is literal or not?  Could this affect some of our modern-day doctrine?

John 21:25  Jesus did many other things as well. If every one of them were written down, I suppose that even the whole world would not have room for the books that would be written.