I managed to miss 'thin' when searching the web site (first hit from where I was happened to be 'thinking' and then I saw the Dell rack servers).

So the "thin clients" (with local storage they look like small PCs to me) are for the hottest content only: "Setting the tiny mirror up only hosting Ubuntu ISOs, Extra Packages for Enterprise Linux, and the CentOS repo for servers easily exceeded our design objective of >1TB/day of network traffic. Not a replacement for traditional "heavy iron" mirrors that can host a longer tail of projects, but this is 1TB of network traffic which we were able to peel off of those bigger mirrors so they could spend their resources serving the less popular content, which we wouldn't be able to fit on the single 2TB SSD inside this box."