Even if there was no "nation-state" of the Palestinian people, it really would not matter. There had never been a "nation-state" of the European Jews either. But in point of fact, depending on how you define a "nation state" the people in the area identified with Pilistu in pre-Hebrew times and subsequent iterations of the name eventually became Palaestina under the Romans. Just because the province of Palaestina was ruled by Rome, doesn't mean the people did not call themselves "Palaestino" (singular) in Latin, as the Romans did.
Furthermore, the Latin Kingdom existed for about a century. The majority of the population were the same people that were always there (many reconverted to Christianity from Islam) ruled by a combination of French and Italian "aristocrats" who were not the first born sons at home. They were also the people of Palestine.
And, the people of Palestine received provisional recognition of statehood under article 22 of the Covenant of the League of Nations.
"Certain communities formerly belonging to the Turkish Empire have reached a stage of development where
their existence as independent nations can be provisionally recognized subject to the rendering of administrative advice and assistance by a Mandatory until such time as they are able to stand alone. The wishes of these communities must be a principal consideration in the selection of the Mandatory."
Avalon Project - The Covenant of the League of Nations