How does Paul try to prove the truth of Keller? Is this effective?

Paul shares all that he knows about Keller in order to prove to Henisch that he did not die on the march. He describes Keller, he tells him that Keller loves Bach and Mozart, and he hates the Romantics. Henisch says the opposite. Paul then plays Beethoven's Arietta from Opus 111 to show that he knows Keller's style, but Henisch once again gives a negative response, saying that Keller's student (Paul) played with more rubato. He then gives Paul a copy of Keller's last recording from Berlin, a recording made in 1934, after which, he sends Paul away.