was not that we can't colonize space, but more that the classic SF view of people setting up space stations orbiting the sun, domed or underground colonies on the Moon and other planets, and space freighters setting up some sort of interplanetary trade (space pirates optional), much less interstellar freighters shipping people and goods between star systems ain't gonna happen barring a miracle that breaks the laws of physics as we know them. Which is not to say it can't happen but there are interesting consequences to such feats.
A lot of the focus in the essay was on human beings settling off Earth. If we go with robots, heavily altered human beings and various other forms of transcended beings, then colonization of other worlds is perfectly possible, as long as we adapt the people for harsh climes. But that's not the point of the essay. Humanity for the most part was evolved to live on Earth and getting us to survive anywhere else is next to impossible or of dubious effort at best.
And then there is the fact that for the energy/time cost of manufacturing widgets on one planet in our system and shipping it to another part, it would be a lot cheaper/faster to simply send the schematic by electromagnetic transmission and then have some manufacturing facility on the destination planet build it there. Moving matter is expensive. Moving information is a lot cheaper. Space freighters, whether interplanetary or interstellar, don't make any sense. Just because it worked for sea ships doesn't mean it works for space ships.
Does Charlie Stross think we couldn't send sentient robots to Mars to build a colony of sentient robots? I doubt it, but that wasn't the point of the essay. The question is whether humans could settle Mars, and he's rightfully skeptical of that. So am I. If anything from this world settles Mars and forms a viable self-sustaining colony there, it won't be human as we conceive of it.