Almost nobody in Taiwan would call it a "province of China" (some would if you redefine what China means, ie not PRC). But as usual standards bodies bend to whoever has power at the moment.
Taiwan is the last vestige of original ”China” and PRC already redefined it when Taiwan (ROC) lost the mainland to the communists in 1949 during the Chinese Civil War.
Legally speaking, Taiwan is a province of China according to both the PRC and ROC. Of course, they both claim that they are the "China" in question.