I'm sorry, what? Are you claiming we have created a livable atmosphere on another planet? Where? Which one? Are you talking about Earth? Humans did not create the Earth's atmosphere. We can barely "fix" the damage we do to it. "Fixing" is not creating.
He is saying that we are currently transformong the earth's atmosphere into a less hospitable one through a process of pollution / emitting greenhouse gases.
In my view, given that we can do this, it is not unreasonble to assume that one day we will know enough to be able to tweak a planet's habitability in the opposite direction, or in other words, make an inhabitable planet habitable.
> "make an inhabitable planet habitable."
Aren't those the same thing? Shouldn't this be "make an uninhabitable planet habitable"?
Given or current sample size of 1, clearly we can.
It's the part where the teraforming produces a more habitable planet that we haven't gotten down yet.
But hey, this is only or first attempt!