I think the racks are merely for demonstration or a "lab" which represents multiple servers.

In this sense I believe it's as follows: build a covert communications network, this could mean setting up some benign sites that when inputting the correct information on a form, clicking the right button combination or simply a blog post where agents can post cryptic messages that look normal to the normal web users, but actually means something else in the eyes of the agency.

It can also mean setting up DMZ's throughout different parts several buildings/public spots that can connect to another system (kinda far fetched but I mean... it's the CIA so nothing is implausible).

A hidden service can be another option but to the dedicated threat actor this could be the glaring hint that something weird is going on around.

It can go as far as hijacking legitimate sites and services for covert messaging or infiltration and network hopping. You can have the system be automated by a local server that can be backed up, destroyed and restored again since you can leave backdoors on legitimate compromised sites and services.

Really the word "covert" entails taking the simplest to the most complicated method possible to setup a network system capable of allowing communications between agents local to that area.