I suspect the ambitious timelines are political. Musk has repeatedly stated that others would need to work on a lot of the Mars colony infrastructure. SpaceX only appear to be working on the supporting infrastructure for flights (e.g. propellant plant).
By telling the world that they'll be ready to fly in five years, they're making a statement that other organisations/companies that want to get involved with this effort need to start working on developing the other parts right now.
If the schedule ends up being more like 2028 for the first flight, they stand a much better chance at flying the right stuff there.