Uh, the level end cutscene here...
Clearly shows you that the Covenant had just "fully" invaded, before the camera is cut again.
I know, that's the famous scene where the covenant armada invades
"detecting multiple targets below the defense grid"
"Impossible, check your source"
But in the original depiction of the Fall of Reach in the novel it was a completely different type of battle. In the game the UNSC know that the Covenant is in Reach way before the main fleet arrives. In the novel it was a surprise attack done by the covenant, and they caught the UNSC completely off guard.
At the time in the forums of Bungie.net many(not all) fans were dissapointed with the campaign because we expected battles like in the one in the "Deliver Hope trailer". Because in the original interpretation it was a
direct assault on Reach because the covenant already knew what Reach meant to humanity, not only that it was lead by Thel'Vadamee(The Arbiter in Halo 2) which at this time he was one of the most acomplished commanders in the Covenant Navy. And the Covenant absolutely outplayed, outgunned, outnumbered humanity in this battle.
In short, as you stated if you view Reach in isolation, it did do what you said it presented a pretty bleak scenario. But when you compare the campaign to it's source material and marketing it fell short and it got some details wrong which in turn soured my opinion of the campaign as a whole.
Again, no mission feels as hopeless as the "Deliver Hope" trailer. Or as brutal as the Reach reveal was.
Edit:If you notice the Reveal trailer it's the same event from the cutscene at the end of Long Night of Solace.
In the game, Winter Contingency is declared in July on the first mission. On the original dialogue Winter Contingency was declared on August 30 at the same time the covenant fleet appeared on Reach's orbit.