First, I did not say that it is impossible. I am just expecting a crash landing if he sends a StarShip to Mars in that time frame. As others have already demonstrated, soft landings on Mars are more probably failures than successes, even with spacecraft that are specially designed just for that mission. With Musk's "fail soon and fix the problems found" development strategy, I expect him to build a pretty big pile of litter on the Martian surface before there is a reliable lander for that mission.
And, actually, I do have a couple of degrees in "rocket science", so I am thinking that a Mars colony will probably need a variety of vehicles for the interplanetary trip. There will probably be "StarShips" that are used for ferrying people and supplies between Earth orbit and Mars orbit that are not designed to descend to the surface of either planet. And there will probably be orbiting rendezvous points in orbit around both planets for transfer between the interplanetary vehicles and the planet-specific landing-capable vehicles. With Musk's philosophy that "the best part is no part", it seems unlikely that he would design his system so that a single ship would go from planetary surface to planetary surface, dragging the necessary landing hardware back and forth between the planets on every trip.
For landings of people using craft that can relaunch into orbit, I don't think "disposable sky hook" type landers are an attractive option. So, the landers that carry crew between Mars orbit and the Martian surface are going to require some innovative thinking - and I agree that Musk and SpaceX are really good at that. For one-way deliveries of supplies from Earth to a Martian colony, I would not be surprised if the initial solution is disposable equipment between Mars orbit and the surface. Again, optimizing that, starting from Earth and figuring costs in fuel from planet surface to planet surface would be something for innovative thinking.