Are they actually running iOS firmware in a web browser, or is this more like an advanced version of BrowserStack, where the firmware is running on a server or a physical device?
But to answer your question, It is actually running stock Apple iOS firmware on a ARM hypervisor which Corellium has created and they are relaying the screen output onto the web from the hypervisor.
You can think of Corellium as like a enterprise-grade Docker, but for iOS and Android apps.