This was to squish a specific rumor that there was *currently* a sequel in the works. After all the news sites took his quote out of context and started preaching the end of days for Firefly/Serenity, Joss came back on Whedomesque to tell them to chill and explain the situation further.
whedonesque.com/comments/11513#144407

Of partiular note in the last paragraph:

"Here's a thing: when "Firefly" was cancelled, my heart got broke. Sounds a bit much, but it changed me. Not even "Serenity" could patch that wound. I'm wearier, warier -- after all those years as a movie writer, you'd think I'd be prepared for another lesson on my unimportance in the scheme of things, but I wasn't. There are two very separate worlds: the marketplace, and the bustling bazaar that is my brain. The brain place is crowded with goods, ideas, sequels, spinoffs, animated versions, miniseries, radio dramas -- this is just the used goods. All the new wares are in there as well and it's deafening. Once I create a verse I never let go of it. And figuring out how much of my energy should be devoted to reawakening the projects you all love with the actors and characters I all love, and how much should be forging ahead and creating entirely new works (which you are contractually obligated to love) is exhausting. More than you know. You know the horse caught bwtween two pools of water? Add seven pools, and make the horse wicked A.D.D. The other world, the marketplace, I don't even begin to understand or predict. All these rumor of projects or the death of projects... When the two worlds align and something actually happens, whatever it is, you guys know I'll be on this site as soon as I'm allowed to be. And I'll be very very clear. There is no news. Not never, just now."