Personally I see it as the author being vague to avoid conflicting with established dates rather than very subtly hinting that the Red Corsairs have been active for longer than previously established.
Oh, we can get pretty close.
It happened around 994999.m41--The Fall of Vilamus.
Theoretically, I guess, the fleet from Hell's Iris (the space station) could have taken a long time.
We DEFINITELY know it's "sometime after the Badab War" (which ended in 912.M41) and sometime BEFORE the 13th Black Crusade (999.M41).
I didn't realize/remember the Maelstrom had planets in it.
"They’d already sailed past several worlds, through the heart of three systems. On one of the worlds, the oceans had boiled, visible even from orbit. Unnatural storms plagued the planet’s face, raining piss, acid and blood onto the continents below."
Huron has somehow increased his force from ~200 (survivors of Badab) to ~100,000 (average Legion size)
I don't think we've seen numbers to support THAT many. I'm thinking closer to 10-20k. But if someone could provide more solid numbers, I would be happy to read their sources.
The Astral Claws were going absolutely nuts in cultivating new geneseed, before before and during (perhaps especially) the Badab War. They also stole whatever they could (from ally or foe).
I can't imagine Vilamus is their first geneseed grab--though almost undoubtedly their most successful.