yes there are methods which are not listed in the API documentation. The reason why these methods are not public is that they're most likely going to be changed, replaced, renamed or even removed. If now users are using these methods in their scripts, it can be that their scripts will break with a future update. In this case it's very difficult for the user to find out what happened.
On the other hand the methods listed in the API documentation should not change. They can be deprecated, but they should still work. Using these methods the script should always run even if we update the API.
Regarding the second question. Yes you're right :) The item.animationState() method is pretty useless without other animation service methods. But this will change in the future.
The CoSpaces Scripting feature is still in "Early Access". That means that there can and will be a lot of changes in short time periods.