The "entire engine" wasn't ported for that demo, it was a tech proof of concept using "UE3 tools and technologies". It was at best a vertical slice, no physics, minimal logic. But the focus was the rendering, and that was impressive. I am not saying what they did wasn't cool, but the "4 days" part is largely PR. They already stated they they (and mozilla) had been working on it for several months, and it was even a couple of months after they showed it off at GDC before they actually put it up on the web. And don't forget the Citadel demo had been already been ported to flash two years before. They didn't just decide one day to start and 4 days later had a complete project. Two companies, creative developers and couple of years of work lead to them being able to compile an existing limited demo in a few days. Don't be fooled, a lot of GDC demos are vertical slice/smoke and mirrors, it is about marketing/building hype, when it ships, it is real. The demo was cool, the new stuff is very cool, it will be interesting to see how it evolves.