The area needed depends somewhat on where the panels are located, the north/east don't see a ton of sun for about half the year. Also, 70+% of TW is covered by mountains and dense forest. I don't mean to say it's impossible (I'd like to see a lot more solar), just that the economics aren't as good as other places.

I also wonder if building for typhoon conditions affects construction costs significantly.

$10b seems impossibly low to go fully off-grid, I expect real estate costs would dwarf panel/battery/construction costs. TSMC isn't stupid, so I assume they've explored the options in this area.