Empire takes place after Betrayer. Can't find the exact passage, but Guilliman comments on the "horrors wrought upon Angron by Lorgar." Plus, the ruinstorm from Calth didn't eclipse all of Ultramar immediately, hence why Guilliman could navigate back to Maccrage before they actually figure out how to use the Pharos as a pseudo-Astronomicon.
I'm really getting tired of the mary sue they're making Curze into. Previously, the Lion single handedly cripples him in single combat. Now, he's somehow able to withstand combat from two primarchs at once, then continually take huge amounts of damage from an unkillable Vulkan, get trapped by a daemon in the warp, and still make it out to faff around on Maccrage. Bullshit.
As far as the siege of Terra lore goes, I think they'll hugely depart from canon just to keep the series from ending as long as possible. Were I to guess, however, I'd wager that either a.) Curze does something to majorly delay the Ultramarines from leaving to reinforce Terra with the Blood Angels and Dark Angels, b.) some issue with the Pharos allows only one of the three legions to make it home, c.) Guilliman decides to "bring up the rear" so to speak in order to destroy any reinforcements Horus might have coming, as well as fight off whatever forces that try to delay the Blood Angels, or d.) the Dark Angels go do their own thing elsewhere; maybe a retcon that sends them to Caliban instead of Terra first. Possibly all of the above.
I also thought Lorgar was at the Siege of Terra because of the massive daemon summoning that went on there; it was always the Word Bearers trademark. I know for a fact that Mortarion, Angron, Perturabo, Fulgrim, Horus, and Magnus were all there.
One weird thing they just did in canon with the new Vengeful Spirit is place Leman Russ on Terra after he got away from the Alpha Legion ambush. He's there talking to Malcador, and about why he's not staying for the siege. Canon always said the Wolves couldn't make it in time because of the Alpha Legion delaying them, but now he's supposedly running off to try and kill Horus. I find that to be complete and utter bullshit retcon that makes no sense whatsoever. It's purely intended to give them another twenty goddamn books about the Space Wolves running around in space doing shit that won't matter at all in the end.
GW is going to end up retconning everything after Calth at this rate. They can go fuck themselves, greedy little shits.