The fight occurred somewhere in the Eye of Terror. We are not given a date but it was after the Scouring as Lorgar had already ascended to Daemonhood (which make Corax kicking his backside even more impressive). Given that Lorgar has spent most of the last 9000 years in seclusion, and that seclusion starts at the end of the story, I think it is fairly safe to assume the story takes place fairly soon after the end of the Scouring.
Corax says that his new powers are the result of being in the Eye. The closeness to the Warp reveals the true nature of the Primarchs. We know from earlier
HH novels that the Primarchs are more than just masterpieces of genetic engineering. Sorcery also went into their creation and they seem to be bottled warp energy. This explains the more exotic abilities some of the Primarchs exhibit, they are more than just flesh and blood.
What would happen to other Primarchs in the Warp is not made clear but possibly something similar. Russ also vanished (possibly into the
EoT) so Wulfen Russ remains a possibility. Whether Corax's new powers would remain if he were to leave the Eye is also not stated.
The main bit that amused me is the fact that Lorgar has not really spent the last 9000 years worshiping Chaos, that is just the Word Bearer propaganda. He has spent that time hiding from Corax.