“Arab” can have a variety of meanings. The author is trying to imply that there are no real Palestinians, and therefore have no real claim to Palestine. They are Arab speakers who are indigenous to the territory known as Palestine and are thus rightly called Palestinians. They are not, Jordanians, Egyptians, Iraqis, Yemeni, etc. Although they all speak Arabic, that are all distinct peoples, just as are the various English speakers around the world.
Not a unique race, but a unique people. The author is trying to push the idea that there are no such things as real Palestinians and thus have no real claim to Palestine.
"Indegeneity" (or is it indigenousness?) doesn't exist. The only human being who came directly out of the soil was 'Adam HaRi'shon (but you believe in evolution, right?). There are only people who were already there because they invaded it earlier.
Besides, the only thing that matters is to whom G-d gave the land. No other claim matters.