Yeah I got to that, too, and it's not correct. A given CO2 level basically determines the temperature at which the Earth will be in net radiative equilibrium between heating and cooling.

The Earth can't simply continue heating up until the oceans boil and the rocks melt because the CO2 level has gone up a bit. That would only be possible in the presence of a very strong positive feedback loop that made the Earth better and better insulated the hotter it got, which would be unlike any other thermal system we know of.

Edit: there are some positive feedback effects, like water vapor being a greenhouse gas, that will likely make the end result hotter than that expected based on the change in CO2 alone, but it will not lead to heating just continuing indefinitely.