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:
Looked at their source code out of curiosity. They use Elasticsearch as a geo backend, tile server that renders PNG images server side for each map tile request.
- low zoom -> server aggregated grid
- high zoom -> switcher to point tiles
browser just displays images, there is no work to do on client side..