I suspect whether or not something is a pollutant, much like whether or not a plant is a weed, depends on context. In some countries, blackberries are welcome; in mine it’s a pest, with no natural limiter. It pollutes an environment and disrupts it’s normal processes, therefor it’s pollution. It doesn’t have to be nefarious or useless for it to be considered pollution, even if we have case-specific definitions like weed and pest. Fortunately, we can offset CO2, just like we can pull up blackberries (no easy task for either), but they do make a difference.