That literally only happened because the USSR and China would not and could not vote on it (respectively) because the UN was insisting that the KMT who only controlled taiwan were the legitimate representatives of china, and the USSR was boycotting the UN votes on principle.

The point isn't "UN is infallible" the point is "Even anti-communist countries in the UN agreed that PRC is the legitimate government of all of China."

Claiming a territory against the will of its people is exactly what imperialism is

In case you're interested in learning more about imperialism from a Marxist perspective, I'd recommend Lenin's Imperialism, the Highest Stage of Capitalism. You don't have to agree with it, but I don't believe you can have a complete understanding of the way Global South countries have been exploited and subjugated in the last century or so without being aware of the arguments made in this book. The fact that so many of the post WW2 anti-colonial liberation movements were ML or MLM should tell you something.

Also, China doesn't have to claim anything. Elsewhere in this thread people have already pointed out that under International Law Chinese Taipei is already part of China. No country in the world recognizes the ROC or Taiwan as an independent country. To the extent that the populace of the island has a position on this, it's split between wanting to retain a special status within China or separatism. And let's be real here, no country in the world is down with separatist movements. Ask the Kurds, or the Basques, or the Catalonians, or the Chechens etc etc. Hell, let Texas try and secede and see what happens. Realistically, only once Capitalism and Nationalism are both dead and forgotten can humanity start to move away from this geopolitical reality.

Claiming a territory against the will of its people is exactly what imperialism is.

It is nothing less than colonisation.

You mean like when the KMT fled to Taiwan, brutalized the natives, and colonized it, imposing four decades of martial law?

Lenin undertook his detailed study of Imperialism: The Highest Stage of Capitalism in 1916, basing it on the research of an English economist named Hobson. His analysis continues to explain what is happening in the world today as we enter the 21st Century.

Lenin saw capitalism evolving into a higher stage. The key to understanding it was an economic analysis of the transition to monopoly: "...imperialism is the monopoly stage of capitalism." As Lenin would point out in another article written in 1916 (Imperialism and the Split in Socialism), imperialism was a new development that had been predicted but not yet seen by Marx and Engels.

Lenin provides a careful, 5-point definition of imperialism: "(1) the concentration of production and capital has developed to such a high stage that it has created monopolies which play a decisive role in economic life; (2) the merging of bank capital with industrial capital, and the creation, on the basis of this "finance capital", of a financial oligarchy; (3) the export of capital as distinguished from the export of commodities acquires exceptional importance; (4) the formation of international monopolist capitalist associations which share the world among themselves, and (5) the territorial division of the whole world among the biggest capitalist powers is completed. Imperialism is capitalism at that stage of development at which the dominance of monopolies and finance capital is established; in which the export of capital has acquired pronounced importance; in which the division of the world among the international trusts has begun, in which the division of all territories of the globe among the biggest capitalist powers has been completed."

the bourgeoisie are increasingly compelled by a falling rate of profit to use their dominance of the state apparatus to open new markets or access to resource extraction

Last I checked liberals are all about respecting rules and international law, so we're just holding you to your own standards here.