It might be that there's not enough sealed environments and gas masks to go around for everyone and the slaves are last on the priority list, of course,
You can bet that they are last on the priority list. Also, we're not speaking about any Amarr here, but Sarumites. If it's strategically and tactically the sound strategy to get property they can't reasonably defend out of the way, because it will be otherwise be dangerous itself, then, well, pragmatics says kill them off. It's not that it'd make the situation better if you'd have to shoot them once they attack you...
but even then the tone of the article just seems.. off. I'd expect the expected miserable deaths due to chemical attack to be mentioned somewhere.
I
expect them to be made as well. I'd also
expect that Elder's don't waste valuable insorum if they won't reach any slaves with it. And I'd
expect that if a chemical agent that is supposed to be leathal is used in orbital bombardments that it would come up in the news that it actually isn't leathal.
Having re-read the writeup for that year in Evelopedia, it seems though that, indeed, Insorum was not leathal. It is as well alluded there that on Sarum lots of slaves were captured by the Elders. How this is actually consistent with Sarumites killing the slaves off remains a mystery. Maybe they recovered the dead bodies.
Even so, news articles and writup agree that Insorum, even though not itself a bioweapon, changed apparently slaves affected by it:
some kind of directed chemical {later identified as Insorum} bombardment was delivered from low orbit, provoking rioting and mass uprisings among the slave population
So, okay, maybe Insorum wasn't directly a bioweapon, still the Elder's had no quarrel with turning the slaves into such. Not sure if that's really better.
I still think there are lots of unresolved questions and contradictions in the material there.