As a Taiwanese American guy, I definitely know that my biological mother speaks Taiwanese and Mandarin separately. Mandarin is used mostly by the Taiwanese folks who are derived from the Chinese invaders whereas Taiwanese is used by the native Taiwanese people who were not part of the descendants from the Nationalist Army.
It's fascinating to see how outsiders attempt to interpret a situation which requires a firm understanding of a 20th century Taiwanese history/political situation, then fabricate or surmise some type of scenario without the prerequisite research.