If the Terminals are all about Thel Vadam, why would there be information about the Covenant supreme commander on a previously undiscovered Halo? In its Library?
Or might they just collectively be a meta-narrative device allowed by the fact we're playing as Locke using some kind of simulator to see what John did? Or Locke's watching our playthrough on a hologram, as per the E3 trailer, and looks at another screen for the Terminals?
Guess we'll have to wait and see.
Fun* fact: before they announced the focus of the Terminals, I completely failed to put two and two together and realise Thel was at the fall of Reach.
*Not actually that fun, just a bit stupid, really.
-
I think it's simply the same as CEA. The terminals are there to provide backstory/information that we never had before. Just to expand the narrative. In reality they're not actually there. They're only there to represent a fun way to learn more about a beloved character, rather than a simple video you would find on the main menu.