In principle, yes, and I do think this is an underappreciated issue which, if understood better, could have avoided several needless heated arguments in general.
However... Where does your understanding come from? It runs contrary to my experience from living in the region.
IME, "Mainland China" is one of the least loaded and ambiguous terms, and tend to refer to "the literal mainland of the PRC" (that is, sans Macau and Hong Kong, and definitely Taiwan).
> When you say "I am from mainland China", there is a good chance that you do see Taiwan as a 'rogue province' and may support its 'inevitable' reunification by any means, including force. That latter sense is the one that is highly political and irks many people.
While technically, yes, but it could equally (and probably more often in practice?) be used by people who recognize the independence of Taiwan and do not want to force a "reunification".
"China" (and even more so "Chinese") is more open for being problematic as it can imply the citizens, ethnicity, cultural heritage, the CCP, the PRC nation-state as such, etc... Being more specific removes room for misinterpretation, willful or not.
This is my perspective from living in the region for a couple of years. Though I wouldn't say I've knowingly interacted with more than a small handful of "pro-unifiers".
Taiwan's official name is the Republic of China, so the folks I've heard use the "mainland" qualifier aren't saying "there are two parts of China" but rather that there are two countries that call themselves "China".
Besides, if ambiguity is the concern for people from the PRC, the phrase "I am from the People's Republic of China" lacks any political subtext.
The reason why many Chinese, regardless of nationality, use the phrase “mainland Chinese” or “mainland China” in English is because it’s the direct translation of 中国大陆, the phrase used to identify the mainland. It doesn’t indicate political affiliation in any way.
* They were taught to do so and simply understand it to mean 'someone from the People's Republic of China'. No political subtext intended.
* They use it to mean that they are from the literal mainland of the People's Republic of China, but not Hong Kong or Macau.
* They use it because they genuinely see Taiwan as 'that other part of China' that needs to be unified with the People's Republic of China, despite having been a sovereign democracy for over 70 years.
When you say "I am from China" (or "I am from the PRC"), there is no chance of this having any deeper political meaning. When you say "I am from mainland China", there is a good chance that you do see Taiwan as a 'rogue province' and may support its 'inevitable' reunification by any means, including force. That latter sense is the one that is highly political and irks many people.
Note: there is no need to get into the justness or wrongness of Chinese reunification here; it would be off-topic and not likely to be fruitful. I do think it is important that people understand that the phrase 'mainland China' is not bereft of connotations and may cause readers to form an opinion of the person using it that may not be in line with that person's actual intent.