You’re right, being a bad negotiator does not always correlate to being a bad communicator. But being a good negotiator almost always correlates to being a good communicator.
I don't agree with this. I had a great salary negotiation, over the phone. It was all prepared, i had answers ready for me for the negotiation, prepared by my stepsister (who recently had a mission at a recruitment firm), she listened to the interviewer/manager, pointed me to the correct idea for negotiating, and i landed +60% of my old salary (loosing two vacation weeks, i only have 8 left, but still). Nothing to do with being a good communicator.
I did say “almost always”. Also you showed (somewhat morally ambiguous haha) problem solving and resourcing—two highly sought after skills.
Let’s also not discount the fact that negotiation is a skill itself, and you were learning from your stepsister who sounds like a skilled negotiator. If you had to negotiate another raise in-person I would bet you could think back to the cards and knock it out.