> Humans can live off the land just about anywhere on Earth
99% of humans only can live off the land because of technology. Most of the US would die tonight without man-made heating, clothing, and shelter. Mars would require different technology, to be sure.
> For colonization to work, it must be self-sufficient nearly from day one
Why? The American colonies did a lot of trade with other countries. They have never been "self-sufficient."
A Mars colony would need to place priority on low-density objects, like generating their own food and water. We obviously haven't done it yet, but we don't think it's impossible. They would still use trade to get in very wealth-dense things, like nuclear fuel and computer equipment for a while, but that doesn't mean that they would necessarily be charity cases.