… road map?
Builds using items that are not released yet but close to at least and it could be a special dev planet server which allows things a normal creative planet can not and heck even test things that are server side for the most part.
They could have turned on the Lua scripting on this one for instance and do all kinds of fun things.
I can also imagine hidden info scattered around… A giant puzzle hunt thing, not sure how that is called in English…
Basically the devs showing some things of the future in a way they are best at: thru their programming and being creative!