This is exactly correct.
> I think it's idiotic though (because regions are 100% separated within an account
But still bound to the same service limits, no?
I'm not sure what you are referring to (pricing or service quotas?, I will assume the latter), but I think it depends on the service. For some setting the service quota will be per region and for others it will be global.
One I know that is for sure regional is the limit reserved concurrent executions of Lambda functions.
Service limits for global services like Organizations appear to only be manageable from us-east-1.