I was really surprised this is technically possible on iOS, so I watched the demo video again and noticed the hardware plugged in and a “screen broadcast” going on. The dongle is mentioned [here](https://www.ycombinator.com/companies/blue):

‘On iPhone, a tiny USB-C plug-in (“Bud”) provides precise input control’

Really cool they got this to work and it seems great for accessibility. I wonder how Apple feels about it