What they tried to say is, that Taiwan also considers mainland China to be their rightful territory. Taiwans official name is Republic of China, Mainland China’s official name Peoples Republic of China.

Both consider themselves to be the rightful government of the whole China (including both mainland China and Taiwan). Both do not consider the other parties rule to be legitimate.

It really is comparable to Korea or pre-unification Germany. Both governments are united in following the “one China principle”.