I think he was comparing S3 to serving the image assets from his application servers, where if he uses a CDN like MaxCDN, the app severs are only used as origin

I think he was comparing S3 to serving the image assets from his application servers, where if he uses a CDN like MaxCDN, the app severs are only used as origin servers and the load is minimal. This assumes you have the disk space on your app servers for ~750gb of data, though. I actually agree with him - we use MaxCDN/NetDNA at WPEngine and they are awesome (although we don't use our app servers as origin servers).