A statement of fact of the form "A threatened B if B would not comply with A's unlawful demand; B refused to comply with A's demand; A killed B." is not a statement of justifiable homicide, but rather the factual statement on which a prosecutor can seek conviction for premeditated murder. In this case A is Hitler and B is Poland.
Pointing out that had B negotiated with A his murder might not have been a forgone conclusion does not relieve A of indictment for first degree murder. It is merely analyzing B's possible moves in a game theoretic sense.
He does precisely that, by blaming the horrors that followed on the Poles' failure to negotiate: "The cost of the war that came of a refusal to negotiate Danzig was millions of Polish dead, the Katyn massacre, Treblinka, Sobibor, Auschwitz, the annihilation of the Home Army in the Warsaw uprising of 1944, and 50 years of Nazi and Stalinist occupation, barbarism and terror."
It's a historically silly position that Hitler would have played nice, except for the Poles' intransigence, but that's what Pat is saying here.