Palestine was considered and treated as a Class A Mandate, a separate and distinct territory under the League of Nations Mandate, and distinct from Jordan. Palestine's independence had been provisionally recognised in the Covenant of the League of Nations. See "The Origins and Evolution of the Palestine Problem," which can be obtained on the website of the United Nations Information System on Palestine (UNISPAL).
The British Mandate acquired jurisdiction de jure over Palestine in September 1923, following the conclusion with Turkey of the Treaty of Lausanne. On the request of the British Government, the Council of the League of Nations passed a resolution on 16 September 1922 reaffirming that Palestine was a distinct territory.
Palestine had been part of the defeated Ottoman Turkish Empire. The "Mandate for Palestine Articles," elucidated by the Council of the League of Nations on 24 July 1922, refers to "The Government Of Palestine" explicitly in Article 28 and throughout the said document. The Macdonald White Paper from May 1939 envisaged a termination of the Palestine mandate by 1949, with independence for the country in which Palestinians and Jews would share in government.
The unrest in Palestine culminated in the UN Partition Plan Resolution 181(II) of 29 November 1947, where a Jewish state and Arab state would be linked by economic union. A day later, future Israeli Prime Minister Menachem Begin rejected partition and stated unequivocally that "all of Palestine, from the Mediterranean to the Jordan River will be restored to Israel." This excluded Jordan, which had already attained and secured independence from Britain on 22 March 1946.
The subsequent proclamation by David Ben-Gurion of the new State of Israel on 14 May 1948 saw the Israeli delegation to the Conciliation Commission for Palestine specify that its "political frontier should remain the same as that which had existed formerly between Palestine and Jordan" prior to May 1948. So there was an obvious awareness on the Israeli side of the distinction between Palestine and Jordan.
The argument that Jordan's population consists mostly of Palestinians and that, therefore, a Palestinian state already exists is both disingenuous and cynical. The Palestinian presence in Jordan results directly from their expulsion from Palestine in 1948 and the West Bank in 1967. Under such an argument, one could say that Britain is Australia, but no one would call for Britons to be transferred there because such a claim is totally baseless.
Therefore, the idea that "Palestine was Jordan" or "Palestine is Jordan", is legally, historically and politically invalid.