Do you know what is ROC? it is "Republic of China". So what are they if you don't call them Chinese?
And why do we, Chinese descendants from all over the world still call Ourselves Chinese? We are not People Republic of China nor Republic of China's Citizen, but we call ourselves Chinese, and other people call us Chinese, too. So why is that? Why nobody call us as Han, Manchu, Mongol, or others? But instead they call us Chinese?
That's because the term of Chinese is no longer to be considered as a mere citizen of a country called People Republic of China or Republic of China. But also as Identity and culture, and of course, ethnicity. It started with Sanguinis Citizenship belief in the past. But today, it become more of Identity, culture, and Ethnicity. Basically we don't care about PRC as a country. Even if CCP is collapse tomorrow and the country become another type of country, We still call ourselves Chinese.
So
@Menthol is not wrong; he's right. They're Chinese who live in Taiwan. Not a Taiwanese. While people who live in China Mainland are Chinese who live in China. Because there is no Taiwanese, only Chinese.