Moreover, I doubt that the use of Mandarin was widespread in Taiwan before 1949 when Kuomintang occupied Taiwan.
Before that, most people from Taiwan were either natives, who spoke an Austronesian language, or Chinese immigrants who had come during the previous 4 centuries from Fujian, who spoke a Hokkien dialect very different from Mandarin.
Yeah if you want to shut down a conversation in Taipei, or suddenly run into "oh no I don't understand your English," bring up the treatment of the native Taiwanese.
To be fair to the ROC, the Japanese colonial era was FAR worse to them than anything that happened post 1949, but still.
Before that, most people from Taiwan were either natives, who spoke an Austronesian language, or Chinese immigrants who had come during the previous 4 centuries from Fujian, who spoke a Hokkien dialect very different from Mandarin.