They could host their own caching proxy between OSM and their users though.
Also, Home Assistant uses OpenStreetMap and they have more users than Immich does.
Edit: Home Assistant does use OSM data, but they use it via another third-party called CARTO, who at least have a proper site: carto.com/basemaps. Tiles come from URLs like https://basemaps.cartocdn.com/rastertiles/voyager/12/657/[email protected]