bike pollution is slightly more than nil just because of the CO2 we breathe out while riding

Technically, the CO2 animals exhale is carbon neutral because it's from plants you eat (or your food eats). Unless you're eating petroleum derived products of course.

I say technically because while the plants themselves are carbon neutral, modern food production and distribution, especially meat production, still has a large carbon footprint. So your breath is only truly carbon neutral if you foraged for food in the forest on foot.