Political parties don't control territory. Nation states do. The island of Taiwan is a province of the nation state of China - it was ceded to the Japanese imperialist who invaded China and took Taiwan from China making it part of Japan (not part of a specific party in Japan) and then the nation state of China, collaboratively amongst its political factions, liberated the island and restored it as territory of the nation state of China.
This talking point about the CPC never owning it is a category error and you would never imagine making such claims about Labor or Conservatives owning this or that part of England, or Texas being the rightful property of Republicans or New York being the rightful property of Democrats. This use of language is double speak.
As for defending against US aggression being whataboutism, that's not even close to being n accurate use of the phrase. For example, when Russia stationed nuclear weapons in Cuba, the US threatened total nuclear annihilation because it was threatened by what was a consensual agreement between two independent nations. The US making it very clear that they were going to bring their military might to Asia, while expanding their drone war and demolishing a half dozen countries is a far greater threat than the USSR ever was to the USA, but for some reason you bootlickers want to pretend that the US moving 2/3rds of their naval assets to the Pacific theater and constantly writing strategic assessments about whether the US could win a nuclear war if it encircled its opponents with sufficient offensive and defensive capabilities. The US's interest in Taiwan is nothing more than as an unsinkable aircraft carrier that produces computer chips (which, btw it has been trying desperately to bring into the US so that it can use Taiwan exclusively as a war zone).
Whataboutism would be saying that China is allowed to kill 500K Taiwanese children because the US killed 500K Iraqi children. Realpolitik is recognizing that China has a requirement to be hyper vigilant regarding everything on its coastline and nearby waterways and islands because the US and UK have been abundantly clear that they are not happy that China has recovered from the Opium War and the subjugation that followed and that the imperialists have no problem with killing people anywhere in the world if it means advancing the interests of their ruling class.