One things that can be confusing/frustrating to many people about geopolitics is that there's a substantial body of "agreed upon" fiction. These are positions that are held officially by numerous, competing state actors that, while factually untrue, hold value to each.

The status of Taiwan is one. Israel's nuclear status is another famous example.

While the common sense reaction is "everybody knows it, why pretend?" those fictions generally exist because they help maintain delicate balances and order between parties that are generally at odds with one another.

What it comes down to is that it'd be lovely if we could all just say openly and outright that mainland China and Taiwan are separate entities, because, in practice, they are. But it's also pretty swimming for the global community for the PPRC and ROC to both claim that they're the real China while everyone does business with each other anyway. So while it's true that they're distinct and it's also true that everyone knows that to be the case, everyone also wins out to varying degrees by officially pretending that it isn't the case.