The Cairo Declaration was merely a statement of intent. It did not constitute a cession of Taiwan territory to China.
Momorandum: Conversation, in the Dept. of State
Dates: January 6 -13, 1951
Subject: Korea, China, and Formosa
As one of the victor powers we have residual rights there until a Japanese peace treaty has been made. The Cairo declaration manifested our intention. It did not itself constitute a cession of territory.
[ source: United States Department of State / Foreign relations of the United States, 1951. Korea and China Volume VII, Part 2 (1951), page 1481 ]
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[Chinese version] https://www.twdefense.info/trust3/frusch-1951aq.htm