What I said in the article was that most devs should have had time to do both, BUT if they really had to pick one, pick mobile. I'm not saying ignore desktop - I'm saying it's the #2 priority.
How well does the "mobile-first" solution work on a Nokia N95 browser? (let's not forget that not everyone visiting a webapp is doing so from latest version of Chrome, FF, Safari, Opera, etc.).
While I understand your concern, it feels misplaced given the context (Rails Rumble). Of course. Unless it is a front-end heavy hackathon, it is only natural that the business problem being solved has more weight that whether or not the site is functional on mobile.
I do agree with the general sentiment that we should consider mobile. But desktop isn't dead. (I assume you probably didn't write your submission on your phone).