I didn't say she attacked Graendal, but she did use the OP to douse the lights, and that is according to Graendal,s pov.

Moghedien was holding the OP the entire time that she was in Graendal's lair. Graendal was still able to use Compulsion on her, even though she was holding the Source.

Sammael thought holding the OP made you immune to Compulsion, and Graendal disproves that.