@atripp --registry is just the global arg for swapping out your registry on a per-command basis (instead of in a config file).
But! i just realised what was wrong, apologies it's all on me. I still had the old url hardcoded in the publishConfig.registry key in the test modules package.json. i didn't realise that would make npm completely ignore the --registry arg, and blindly try and publish to the old registry address! which was why npm install etc were all working totally fine