This is not something we support out of the box with ArcGIS Online. We do support client side clustering of up to 50,000 points and working on a server side implementation. To create something like the sample provided you would have to explore creating a custom application.
Upon further investigation of the application provided it appears to be developed using Leaflet. It takes advantage of tiles for fast drawing speed and uses the Leaflet.markercluster plugin.
We do have Esri Leaflet, A lightweight set of tools for using ArcGIS services with Leaflet. In this API we do have the following plugin:
L.esri.Cluster.FeatureLayer | Esri Leaflet
For any further questions you may have I'm tagging the Developers space
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