>>12179041
If anything it could be argued Idharae was understrength from fighting Hive Fleet Naga alongside Malan'tai and Iyaden.
Aside from the whole "Doom of Malan'tai" does any fluff on Craftworld Malan'tai exist? I know it was pretty much made up for that Nid Codex story, so I doubt it.
So the Invaders were beating up on a weakened Craftworld.