Two weeks of work isn't all that surprising actually, since according to DoubleGJ it was spread out over a while. Assuming you only work on it an hour a day, that's 14 hours of work, and a person experienced with JCS can get A LOT done in that amount of time, especially with a good plan. If you look at that screenshot, the level (while gigantic) has a lot of solid walls and large cavernous spaces. This means layout-wise, the level probably isn't mind-bogglingly complex or anything. Also, you'll notice that much of the eyecandy is copied and pasted, which saves a lot of time (and this is a good thing, since it's only something you can notice when the level is zoomed out like that).

So in other words, it's a very impressive screenshot, but finding out that it is a product of two weeks of work is pretty normal. After all, the level isn't done yet, and it looks like it still needs pickups and enemies, and of course testing (which always takes forever because you have to play through the level like 100 times).