He probably buys assets, music and models before the update thinking about the future: if the update 0.8 is gonna be in a submarine he probably starts to search diving suits, marine music and sounds, marine monsters, planning the scenes, the dialogues, etc. but about having full chapters complete before releasing the next one I don't believe it, he is releasing 1 update every 4-5 month with high quality; others creators need at least 8-10 months to do the same with less quality and even less content, being only 1 man or he doesn't need to sleep or is impossible.