In 1981, Sun said "The cross-strait issue is a Chinese issue, not a Taiwan issue". This fully shows the views of Sun, a man born and raised in China, came with the ROC government to Taiwan, which is the last remaining territory of the ROC, and continued to work for the KMT nationalist party, as "I am Chinese, and this fight is between the ROC and the PRC."

You're taking things out of context. It was said in a meeting with US and PRC scholars, in which he wanted to point out that the "problem" isn't a problem of Taiwan, hence not a "Taiwan problem", but a problem of China, of whether the Chinese population should live under the communist party's rule, hence it is a "China problem".

Note how I translated it as "China problem" not "Chinese problem", while you translated it as "Chinese problem", hence you fell into your own trap of confusing the ideas of nationality and ethnicity.