Good question - I have no idea. But if I google 'gas-filled tube ethernet', I get a number of hits like http://en.tdk.eu/blob/174150/download/5/smd-surge-arresters-... and http://www.first-electronic.com/uploadfile/2010989552637708.... which suggest that these products are sold for networking purposes to prevent surges. Since this is in a military context, I would hazard a guess that this may be some sort of standard hardening requirement to try to protect the datacenter against lightning strikes, EMPs (such as from nuclear strikes), and general accidents. This may sound paranoid, but then again, so do Faraday cages, and it is the military - it's their job.
The wording didn't sound like he was talking about a gas-discharge tube for surge protection. Also, that wouldn't have anything to do with security.
I think what may be happening is the ethernet cable runs are in sealed tubes running at either positive or negative pressure so that if someone tries to breach the tube and splice onto the cable it would be detected by a pressure sensor.
I'm honestly not familiar with these kinds of facilities, but with a potential "Collateral Confidential" type cable, one could imagine a gas-filled tube being a countermeasure of some sort.
You can pressurize the gas and put a barometer in the tube in order to detect any breach of the tube. This was used during the cold war and the NSA tapped the communications cable anyway by filling the space around the tube with gas as well. Source: James Bamford's books on the NSA.