OpenStreetMap is a prominent user of Mapnik, but it's not an OpenStreetMap project. It was started way back in time for the sake of starting it:

https://mapnik.org/news/update

When it got so far along, OSM started using it in the demo rendering system that backs the 'standard' tiles on openstreetmap.org. Those tiles often get called "mapnik", because Mapnik is a big chunk of the rendering system, but they are really OpenStreetMap-Carto, a map style maintained for OSM.