Points are rendered server-side, backed by Elasticsearch, and served as PNG tiles for each zoom level. Individual markers are only rendered for small sets. Some of the relevant source code:

https://github.com/inaturalist/inaturalist/blob/main/app/ass...

https://github.com/inaturalist/inaturalist/blob/main/app/ass...

https://github.com/inaturalist/inaturalist/blob/main/app/ass...