I've always maintained that

Assertive = Aggressive + On my Team

You are assertive if the community around you thinks you are justified in asking for your way, and aggressive if they feel you are unjustified. We perceive our allies as assertive and our opponents as aggressive.

The GP is correct that "assertiveness" training gives someone tools which are equally useful in defending against a bully as they are in bullying. It's really just rudimentary influence training in the vein of Carnegie or Cialdini, and people can be influenced for ill just as they can be influenced for good.