The chips might be mostly produced in Taiwan, but they are mostly assembled into products in China. If China banned use of ARM chips in products that would disrupt basically the entire smartphone supply chain. Of course, this would greatly harm China too. But they can still use the threat of doing without actually doing it.
The majority of Samsung's phones are made outside of China (eg their huge presence in Vietnam). It can obviously be done and done affordably. Samsung wouldn't do that just for fun. If Samsung can move their assembly out of China, so can Apple.
Don’t forget that Apple is slowly ramping up its manufacturing presence in India, in addition to Vietnam mentioned by another comment, to further mitigate its over reliance on China.
This is more spreading out final assembly to dodge tariffs, both US and Indian. Losing out on assembly is minor compared to moving up component value chain. Queue meme about China making $8 per iPhone 3G (3% of cost). It grew to $100 for iPhone X (25% of cost). Reliance is growing.
Samsung's somewhat exceptional in that they have a lot of domestic high end component manufacturing. But even they've been increasing Chinese manufacturing via ODMs for lower end models while media was talking about their move to Vietnam. I think their high end models still sources ~30% from China.
