Bootstrapping means creating a self-sustaining business. At a minimum, this means you have revenue to cover your running costs. You can only say "bootstrap with less than $10K" if that 10K gives your business enough revenue to cover its running costs. Yet this is what one of the founders has to say in another comment here on HN: We will try first to build a community/traffic and then will look for revenues.
I guess it's not the founders' fault that the article was written this way. But once they've spent a lot of time and money on actually building their community/traffic + the inevitable further development costs, and they start making some revenue, we'll know how much it cost them to bootstrap the business.
Some great tips for getting an MVP together though.