It's that casualties never freaking matter for Chaos (or most non-Imperial factions) in 40K. The traitor fleet and forces had been, presumably, majorly fucked up, but it's not an impediment for them, it never is.

Black Legion never suffers personnel shortages, all of their ships are replaceable and demons can, explicitly, get back after no matter what happens to them. They never even mention that Chaos forces are in any way ever inconvenienced by their loses vs. losing whatever strategic goal they had.

Hack writing, ho!