Growth of a space civilization may not be limited by a constant, but by other asymptotes. For example, if FTL travel is really impossible, the accessible space grows with t^3 for three spatial dimensions, which is way less than the exponential growth that or economy is built on.

There obviously will be some constraints on a planet scale, I was addressing more the idea of planet level constraints being a constraint on humanity itself, which implies that humanity is all still on that planet. That is, I think "constraints on the people living on this planet" and "constraints on humanity" won't necessarily always be the same thing, and a in a discussion such as this, a qualification is appropriate.

You are right that the speed of light is very likely a constraint on human population growth based on exactly what you outlined, and while I wasn't thinking of it specifically, falls under what I meant by "some other constraint". :)

Our growing population will be constrained by resources long before we invent a way to colonize other planets.

Well, that's either a (relatively) short-term constraint, or we are no longer advancing. When talking in the context of Dyson spheres, I think that naturally puts the time frame at multiple orders of magnitude longer than any constraint should hold us back, unless we are actually regressing rather than advancing.

Put another way, when talking about tens to hundreds of thousands of years in the future, I'm not sure even a thousand years from now to get even the most rudimentary of interstellar ships registers as more than a blip.