It didn't "reveal" more to the battle.
and Halo: Reach largely ignored the events of Halo: The Fall of Reach.Revealing that there was more to the battle than the hour-long zurg rush isn't really a retcon. If anything, the events of Contact Harvest are more at odds with TFOR than Reach is, in my opinion.
What IS a retcon is the EXACT time the Pillar of Autumn left the system, and hopefully, the means as to which Cortana obtained the Alpha Halo coordinates. Because honestly, I much prefer that Cortana would have to spend days deciphering the Sigma Octanus crystal data by using the Sword Base artifact as some sort of Rosetta Stone, rather than the crystal basically boiling down to a simple treasure map that, when oriented correctly, will lead you to where One-eyed Willie's gold is. Which Cortana only thought to look at because Chief, a guy who's life basically REVOLVES around fighting aliens, and hanging out on ships between battles, recognized navigation symbols. Seriously, it's like being curious as to how it was possible that a hot dog vendor knew what ketchup was.
It completely changed the Battle of Reach from a massive, global invasion where the UNSC threw everything it had at the largest Covenant fleet yet... to a month long siege where most of the damage was dealt by an unlikely Supercarrier.
After some work, the dates and such do match up, but a lot of things had to happen. Like... more than half the planet didn't know it was under attack for almost a month, or the destruction of Reach being a ploy by Admiral Parangosky to jumpstart Red Flag.
If all Reach did was flesh out the Fall of Reach, there wouldn't be a problem. Instead it completely rewrote the nature of the battle.
Not to mention that there's still other stuff that doesn't quite mesh well together.
By the time the covenant attacked in space in the book the MAC cannons were still up. and yet they never used them to take down the supercarrier. Or any of the other invasion force that arrived after the super carrier went down
Key's talking to the Spartans like they didn't know anything about the invasion, despite the fact that in the game Jun mentions Red Team helping civilian evac ops.
How the hell did Key's or anyone else in orbit at the shipyards not know about the invasion? you can literally see the planet burning from space!
Why wasn't key's ordered to abandon reach? You would expect that someone from ONI would have told him not to come back since they were planning red flag during the invasion.
The dates may match up but the game and book just don't work well together. One of these days we need a total remake of Reach, that either makes more sense with the book. Or replaces both of them.