The premise is a high density of lower cost systems in a data center cabinet, not 1U dell servers. The second part to the quote which was left out of the article referenced how we plan our, organize, and wire the cabinets up. We have a confidence that Apple will use the same external shape for the Mac mini for at least a few model generations. They've only changed the exterior dimensions once since it was released. So dedicating cabinets (we are up to 6 now) just for Mac minis isn't that big of a deal. When they change the design we'll adapt and be set for a few more model generations. It's proven to be a stable enough hardware platform to be able to offer a service for them.
The machines are not perfect and they are not for everyone. But they have the most bang for the buck and work out well for start ups, development, and programs that require OS X. There is obvious a demographic for it as there are thousands of the machines colocated between the various companies.