Was wondering the same thing. I think remembered seeing a post when it first came out that they just wanted to focus on an overview of the grammar, without teaching a lot of vocabulary? So if you complete the tree, you'll know most of the grammar you need to know as far as gender, word order, verb conjugations, etc. But then it'll be up to you to build your vocabulary after that.

I was wondering if stuff would be added at some stage. There are lots of things it doesn't really cover as it is, so there's plenty of room for addition.