I think it is just NASA trying to discourage anyone from going to Mars. Every other probe or lander has detected it up until now. If they find large amounts of water on Mars then they can go the LH2/LOX route for fueling the return rockets.
Good catch -- I'm not in favor of *this* human going to Mars, but had you asked me when I was in my 20s, you would have to leave a message on my machine because I'd have been camped out in line to get on the rocket. The conditions are such that colonists will wind up losing their marbles like the original bubble boy did, and nothing they do will turn Mars Earthlike. An atmosphere needs to be added via artificial bombardment, using frozen nitrogen and oxygen from the outer Solar System. Musk's colony will be like living in Spitzbergen, but without the two month paid vacation somewhere else.