That doesn't make sense. He already knew Ainz was unbeatable, yet Ainz still suggested an alliance with him. It's more logical for him to think of ways to make peace or be subservient but safe to the demon lord than trying to fight. Fighting is utterly moronic considering the power difference, and trying to make Ainz's servants betray him is even more foolish. He had no basis to think they'd be inclined to do anything like that...

I call bad (forced) writing.

If Ainz or Demi wanted them to gang up and fight, they shouldn't have shown too much of their power. That'd incite the humans to rebel because they'd believe they have a fighting chance.