> ethnically, linguistically and culturally Chinese
Again I ask - where is this impression coming from? And for what definition of "Chinese?" Can you please write the specific term you're thinking of in Mandarin to be more clear? In English "Chinese" means too many things, especially with the CPC changing the definition over the last 50 years or so. To be fair, the same thing is happening in Mandarin, so it would help if you could clarify.
Regardless, for most of the definitions of "Chinese," the theory doesn't seem to match reality, nor is the framing correct.
First of all, regarding an ethnological and linguistic concept of "Chinese" (translated many ways), we should reject efforts by the CPC to claim to speak for people that have this identity - which is exactly what they're trying to do. Reject CPC cultural imperialism. Such a claim to Taiwan (and Singapore, Malaysia, New Zealand, Australia, etc places containing many "Chinese" people) should be rejected out of hand.
Second of all, Taiwan is not ethnically or culturally "Chinese" for basically all definitions of "Chinese." It so happens that Mandarin is the language written on government documents, true, though... that's not really "Chinese," is it? It's the language of the Empires of Beijing. There are many other languages that existed in the history of Chinese empires. And throughout that whole time, Empires would choose, based on their convenience, who is "Chinese" and who isn't. Sometimes a lot of languages would get amalgamated into that convenient descriptor to serve imperial purposes. The CPC are the latest to do this, this time in the name of "global Chinese communism."
Regarding culturally, Taiwan really isn't culturally "Chinese," in fact over the last few decades Taiwan has engaged in that process by which a culture develops itself in opposition to another one. See masculine and feminine cultures over the years, the Greeks and the Turks, the Brits and the French, etc. Much of Taiwanese culturally identity specifically forms around rejecting whatever the CPC claims, or riffing on whatever cultural heritage was imported by the Qing or KMT. Sure, some of the same holidays are celebrated, but are all cultures that celebrate Christmas, British? (or, white?) (or..... arabian? considering the origin of christianity) Not to mention the history of Taiwan as a colonized place has injected a wide diversity of cultures. Portuguese, Japanese, etc. There's also indigenous culture which is actively uplifted and many Taiwanese celebrate these holidays, eat indigenous food, etc. Indigenous art emblazons basically every single retaining wall in the country.
Ethnically, there isn't necessarily a "Chinese" ethnicity, again a term invented to describe all the people ruled by a given empire. Of course there's "Han" but that is "Han," not "Han Chinese," a new word used to engage in ethnonationalism. And, it really doesn't describe Taiwan all that well, since again, it is a wildly colonized and immigrated place.
The CPC is attempting to soft-power claim Taiwan through Han ethnonationalism and so I'm quite prickly about this subject.