I'm reasonably certain the "de-facto" wording is used as Taiwan isn't actually recognised formally and fully as an independent country internationally. Thats not to say it shouldn't be, but it acknowledges the current truth.
And I'd like to point out that the ROC and the PRC don't even have the same land claims. The PRC claims the mainland and Taiwan. The ROC claims the mainland, Taiwan, and Mongolia. Mongolia claims Mongolia. There are three separate countries making these claims.
The fact that (some of) their claims overlap doesn't magically mean that the ROC and PRC aren't two separate countries. It just means that they are two countries that claim more land than they control.