This isn't true as a matter of historical fact. The contemporary nation-state of Jordan was established in 1946, prior to the U.N.'s partition plan for Palestine. That ignores the ongoing agreements among the nations before 1946. Read about the British Mandate at https://www.britannica.com/place/Palestine/World-War-I-and-after#ref45067.
"Although Transjordan—i.e., the lands east of the Jordan River—constituted three-fourths of the British mandate of Palestine, it was, despite protests from the Zionists, excluded from the clauses covering the establishment of a Jewish national home. On September 29, 1923, the mandate officially came into force."
Basically, Israel and Transjordan were created at that moment as a 2-state solution, but with no decision yet regarding who'd be in charge.
Basically, Israel and Transjordan were created at that moment as a 2-state solution, but with no decision yet regarding who'd be in charge. This statement of yours simply isn't true. In fact, nothing in that Brittanica article to which you linked supports your view of the situation, which I take to be that the establishment of the nation-state of Jordan (or "Transjordan," as it was initially known) in 1946 -- i.e., that part of the British mandate territory east of the Jordan River -- was meant to be the "state" for the Palestinians Arabs living in that part of the British mandate west of the Jordan River. If that's what you believe (and that's what it sounds to me that you're saying), you're just wrong as a matter of historical fact.