Honestly, if you asked these Rumble devs how many of them even tested in Firefox I think the percentage would be low. Such is the nature of these events - you move fast. And as the post says, the aim is to make an "innovative web application", not a "web application that would be delivered to users in an optimal fashion". It was also a Ruby hackathon - given that it's a server-side technology I doubt the client presentation form was given much focus.

I've made this mistake plenty of times before myself. I've dwelled on the the actual real life issues my concept would face and come away at the end of the weekend with a well-considered concept. But what people want at these events is a catchy, flashy demo that wows attendees and that can be tweeted/Facebooked/etc.

This is part of the reason I don't bother sacrificing my weekend to hackathons these days, but still. You're optimising for the wrong use case.