It's totally a skyblock map! You're in the sky, there's a little island of dirt with a tree on it, and...BTW, I am finding it a constant source of amusement that my pack keeps being referred to as a skyblock map. Um, well...no. I mean, sort of...but no.
What they mean is that your pack plays exactly like a skyblock type map does. There's nothing to explore, you have to build everything, progress is marked by the amount of technology and block types you've acquired, and you have to battle the elements to survive. Survival Island has all of these same characteristics and predates skyblock by years.