Assisting in an attack is one thing. But when the attack force is primarily composed of the "assistance", that's another. Mollies should've contributed a few ships at most.
Also, aiding in defense is a different thing then helping in a full-scale assault. No problem with Hessians flying to Alpha to help defend it from Corsairs. But Hessians flying to help the Outcasts assault the Taus? Not gonna happen. Same area, different reason, different outcome.