I agree. Assertiveness and aggressiveness/offensiveness are completely different things.
My OH is an expert of being "assertive", and he hasn't an aggressive/offensive bone in his body, but he gets things done and gets what he want.
He's always polite, smiling, friendly, with literally everyone, but he's also very well read, intelligence, educated, confident, and will speak/act with authority, rather than shouting or with insults, because he genuinely knows his stuff, and won't say a word unless he knows chapter and verse!
Whether it's a shop assistant, NHS receptionist, traffic warden, neighbour, even friends/family, he is polite and friendly to a fault, but like I say, gets what he wants through confidence and assertiveness. I honestly don't think I've ever heard him raise his voice, not to me, our son, nor anyone else really - he's very softly spoken but chooses his words very carefully.
I think DS has got some of that from DH as now he's an adult and working/living away from us, he has grown a back bone. When we're with him and something goes wrong in some way, i.e. overcharged in a shop/restaurant, returning damaged goods (or just changed his mind) in a shop, he's brilliant and being polite, concise, factual, etc with what he wants, why he wants it, etc - no room for misunderstanding, keeps his cool even when the other party starts being a dick etc. Sounds like he does the same with his flat and furniture etc as he's had several problems with landlord/EA agents, furniture deliveries, etc., but he's sorted it all out himself, just giving us a running commentary of what's been wrong and that he's sorted it. Also same with his work - he's already had a few "kick backs" against certain things they've expected (i.e. trip to a different office without them wanting to pay for travel and subsistence etc), but he's "persuaded" them to do so without getting himself sacked!