Only for cities/suburbs that exist in forested areas. This would exclude most desert and prairie ecosystems (for example).
> Humans living in a smaller area of the planet leaves more room for trees.
Sure, mathematically-speaking, and planet-wide. On a more local scale, though, this is not necessarily the case.
There's also the consideration for relative density of CO₂ sources v. sinks. I don't have any actual data here, so feel free to ignore this, but my hypothesis is that urban areas would have significantly higher CO₂ densities than suburban areas (which would in turn have significantly higher densities than rural areas, and in turn wilderness areas), and this high-density CO₂ would not readily/easily/immediately disperse / would be more difficult for surrounding carbon-sink areas (like wilderness and - I'd argue - potentially suburban areas) to even receive, let alone absorb. You thus end up with big lingering blobs of CO₂, continuously fueled by human occupation, and unable to disperse. Spreading out humans would better spread that CO₂ sequestration load, thus allowing better efficiencies.
This is absurd. If people move away from the suburbs, the trees will still be there, and more if we get into reforestation. Building a suburb generally involves cutting down trees. Humans living in a smaller area of the planet leaves more room for trees.