The article assumes but doesn't state a number of conditions needed to make its conclusion correct. First, it assumes that only planets in our solar system should be considered. Second, it assumes that the purpose for colonization of other planets would be a complete (or nearly so) evacuation of Earth, which no has ever advocated or even thought possible. Third, it assumes that any terraforming would be trying to make a 100% duplicate of Earth. Forth, it assumes that any terraforming would perforce have to involve the entire planet rather than just a delineable portion of it.
Building O'Neill-style habitats in space does seem like a more feasible idea for the near term (100-500 years), but modifying a planet to make it more amenable to human habitation as quite certainly a viable idea. We've been doing it on Earth for thousands of years!
Seems that some always underestimate the adaptability of the most adaptable species of the face of the planet. A major factor of our adaptability is our ability to manipulate our environment or even create a small environment we can survive in.
Obviously terraforming mars is a long way off but that doesn't mean we won't or shouldn't start colonizing now. You start with contained habitats that require resupply from earth but as they grow they can begin to become self sustaining. As they continue to grow they can begin to address the problems of terraforming on a wide scale.
I expect the moon to be colonized at some point but that doesn't mean I expect it to become a second earth. I expect it to be more like a massive space station.
