The context is somebody asking "Mainland US or Mainland China?" The comment you're responding to brought up Taiwan because that's the natural "not-mainland" when you're talking about China.
Almost. Both China and USA have threatened military action in Taiwan and Greenland respectively, but legally the USA and Greenland are not one; Greenland is a territory of Denmark despite having an independent government. Taiwan and Mainland China also have independent governments, but legally both consider themselves China, so it would be like North and South Korea if they had never agreed that they are separate countries now. Recently Taiwan has begun changing their identity as an independent country, and began the legal updates, however this is not internationally recognized because mainland china has resisted it, and frankly few countries want to go against china and risk sanctions or other political action from china. Even the USA doesn't recognize taiwan as separate, officially, although actions speak louder than words, and it is clear that most respect Taiwan's desire for independence and treat them as sovereign.
Sort of, except not really, except yes really. It's complicated.
The China that was a founding member of the United Nations was the Republic of China (ROC), and it controlled both mainland China and what we call Taiwan. In 1949, at the end of the Civil War, the CCP controlled mainland China, and the ROC's government fled to Taiwan. Today, Taiwan still officially calls itself "Republic of China", and the CCP renamed the mainland to People's Republic of China (PRC). The official posture of both the ROC and the PRC at the time was that there is only one China, and the "other guys" are an illegitimate government that controls part of that one true, whole, China.
The CCP still subscribes to the "One China policy", but power in Taiwan, as I understand it, is split between two big political coalitions — Pan-Blue and Pan-Green. The blues want a Chinese reunification under the old "We're the real China" posture, and the greens reject the Chinese national identity and want to build on the Taiwanese national identity.
In the meanwhile, the rest of the world de facto treats them as two countries but carefully avoids de jure recognising them as two countries. Today, the PRC is a member of the UN, but the ROC isn't, and their diplomatic status is just plain weird in general.
There are two governments that contain the substring of "China" and their constitutions claim a single unified Chinese country that includes mainland and Taiwan island, most of the world, seems ok with that.
> Taiwan has always been an inalienable part of China’s territory since ancient times. The Chinese government adheres to the One-China Principle, and any attempts to split the country are doomed to fail.