I think 343i's explanation falls apart because it relies on a cover-up that would be impossible, not to mention completely counterproductive to implement.
For one, I don't buy the idea that a single groundside relay was responsible for all communications across the planet - all ships (and likely satellites) have basic maser and radio communications of their own; how else could they communicate in deep space? The only possible way they could keep the majority of the fleet in the dark would be to create artificial "equipment failures" on every UNSC ship. Even if the UNSC fleet brass could somehow orchestrate such a thing, and keep each of the ships in consistent geostationary orbits so that they can't see what's going on in the other side, what's keeping the Covenant from showing up on their side of the planet and ruining the whole plan? Something like that would probably happen every few minutes as the Covenant ships orbited the planet, unless all of them were also on fixed geostationary orbits. Which seems unlikely, since the Covenant wouldn't know nor care about the UNSC's contrived cover-up; they'd have no reason not to show up on the other side of the planet.
Even ignoring the fact the cover-up would be a practical impossibility, it's hard to see why would they keep half of the planet in the dark if they only wanted to keep Keyes and the Autumn out of the fight until RED FLAG could begin. The rest of the fleet wasn't going to partake in RED FLAG, so why deliberately hold them back? It's particularly strange because the UNSC had no way of knowing a larger Covenant fleet would show up. Shouldn't they be concentrating all their efforts to fight the Covenant who are already present and wrecking havoc on Reach?
It seems that Bungie's intent with Reach and 343i's explanation are at odds with one another - the Data Drops seriously downplay the extent of the invasion before August 30, whereas the game depicts a full-scale invasion going on by as early as August 14 (as evident by the hologram of force deployments aboard Ardent Prayer, a massive Covenant fleet arriving, and large-scale glassing going on). It's clear that with the Data Drops, they wanted to mesh the stories of The Fall of Reach and the game together, but the result comes across as more than a little contrived. To me, a better idea would've been to add some changes to the re-release of TFoR to accommodate the new canon from Reach.