I’m think it depends on the person you’re talking to. If you’re talking to a strong KMT supporter who’s family came here after the civil war or those people that gather and fly the CCP flag around yeah, they use mainland because they believe that The island is in some way part of “China”
But for me it’s always used in a way to refer to China without grouping HK and even Macau in. I know many people in HK use this term as well as they see themselves as different than people from the rest of China. Almost no one in HK would deny they are part of china now but they still view themselves different in a spectrum of way.
Many people only know the history of the island from the Chinese civil war and the retreat of the KMT. But many people’s families on the island like mine have settled here hundreds of years before that. Mine came over 200 years ago from fujian to settle and set up trade, develop and owning land here that is now modern day 中和. So to take a look at the island from only from that part of the history doesn’t give you the full picture of what is taiwan. Many here have built the island before the KMT and Japanese, setting up governments here that was not really even controlled by the Qing dynasty.