I have noticed the same, and if you sit in the middle seat you are blessed with an underseat media box taking up 1/3 of your footspace

That is the shared pc unit between the boxes.

Actually there are far better ways to do AVOD than it is being done on planes these days using a central server, that gets updates on near the gates via a wireless(not nessesarily wifi) pushed out in a central fashion, using shared media access.

I could work and design a system much more complete, and reliable that would weigh much less.
I am pretty sure there is some shared storage, I remember in the bootup seeing a NFS mount being done, but locally it had some at least flash storage and all the linux boot was local.