^ Yup!
Developing a demo takes time. (Lots of it sometimes.) Very often, unfinished things need placeholders to be created to fill in gaps for art assets...parts of the code need to be specifically re/written to ensure that part of the game can move forward even though it's not ready..."filming" may require playthrough after playthrough if things don't go smoothly or glitches appear...
And all of that is time not being spent on the actual development. Sometimes, it makes additional work for people, because they'll then need to go back and undo things that were done for the demo in order to continue developing the actual game. I'm assuming the devs are very eager to meet the new release date.