Xi Jinping told Donald Trump during a phone call on Monday that Taiwan’s “return to China” is a key part of the post-war international order, state news agency Xinhua reported.
“China and the United States once fought side by side against fascism and militarism, and should now work together to safeguard the outcomes of World War Two,” Xi was quoted as saying by Xinhua.
China regards Taiwan as part of its territory and has not ruled out the use of force to take control of it, though the island’s government rejects Beijing’s claim and says only Taiwan’s people can decide their future.
You’re broadly correct, with the important caveat that when the KMT fled to Taiwan, they didn’t relinquish their claim on the mainland. The KMT said “we’re the real government, the CCP is a usurper” and the CCP said “we won the war on the mainland, therefore Taiwan automatically belongs to us because… reasons”
This is futher complicated by the fact that when Japan relinquished Taiwan after WWII it was technically ceded to the RoC.
In any case, the government of Taiwan is technically the oldest of the two extant “Chinese” governments, so it makes no sense for the CCP to say that Taiwan needs to rejoin it.