Totally irrelevant, out of historical context as Islam did not exist in 300-400 BC - before the Arabo-Islamic conquest of the Levant. Senseless to use such a speech to reference today's Arabs who have only recently claimed the name "Palestinian".
Jews were known as Palestinians internationally well before the Arabs took on the title: e.g. Palestinian Airways founded in the 1930s was Jewish, not Arab.
Nobody is disputing the existence of the Roman titles "Palestine" and "Palestinians" which were names given by the Roman emperor Hadrian. Before this, the land was known as Judea.
You'll need to do a little more research to understand the background properly before embarrassing yourself in future.
As for the person I was quoting who admitted "The [Arab] Palestinian people does not exist ": Zuheir Mohsen (Arabic: زهير محسن; 1936 – 25 July 1979) was aPalestinian leader of the pro-Syria As-Sa'iqa Ba'athist faction of the Palestine Liberation Organization (PLO) between 1971 and 1979.
