It uses centroids for any amenity represented by a polygon, including urban parks. But the very large ones are typically not urban parks so are not included in the analysis (see my other answer)

Thanks Ely! And yes, the map misses those. It considers urban parks (tag:leisure=park), but not green areas outside the strictly urban space (e.g., Nackareservated is tagged leisure=nature_reserve). I am still thinking about how to deal with those.