Elon Musk does use a lot of words to express uncertainty in his estimates. If you are working on doing things that no one has done before, then the timeline to success is highly uncertain. He would have to publish some kind of probability curve estimate of when he thinks things will get finished for us to really know how good his estimations are. Also, many times you need a first deadline for people to plan to shoot for, but some parts of the system won't get done in time and that deadline will slip. So when you say you are planning to get done on X date does not mean you think you are likely to be done on X date.