Earlier quoted context omitted.
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.
Taiwan is "not mainland China" in the same way that Greenland is "not mainland USA"
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.