So Comcast is limited by FCC rule to 8% margin on their broadband services and has to sell those services to Cox at cost?
I'm not sure you're using 'precisely' correctly.
“So Comcast is limited by FCC rule to 8% margin on their broadband services and has to sell those services to Cox at cost?”
While not fully mature, the intent of NN is to get there. That’s why they are classifying ISPs as common carriers.
And that is precisely the model for you own local telephone carrier. They cannot limit access by any other carrier to their infrastructure (the local loop and local exchange) AND are limited by regulation to an 8% return.