This "free for a week" (or "Free for a weekend" is a special form of advertisement handled by Steam and used by a lot of companies for their games.
It's usually "play the game for a short time to see if you're interested", only that in this very specific case the "game" is a construction set.
But in all cases you are never expected to completely play through a game or to make something with a construction set in that time - it is always to just check if you would be interested in it and if your computer can handle the program before you buy it.