The problem with the books is that they simply don't hold up to basic logic. A single billion-ton port makes no sense because (a) it would never be built to that scale to start with, rather it would be built larger as need required it, and (b) the amount of traffic and delays trying to channel traffic through a single port would be illogical (if not ridiculous). Thousands of years of history with ports in reality has shown the efficacy of NOT concentrating a nations (in this instance, a planet) traffic through one location. As we have seen since the beginning of the industrial era, as ports capacities are reached new ones are built and traffic (and passengers) re-routed. Space is going to be different in a number of instances, but the need to disperse traffic to handle it more efficiently remains. There are some exceptions out there, but they are usually explained away via politics or limitations regarding places where new facilities could actually be built.
A xenophobic or similar government type that abhors contact would be more the type to have a restriction as described - trying to limit traffic to a single gateway for control purposes. Though a planet with 20 billion people is going to generate a LOT of traffic and trade, thus merchants are going to demand more efficiency than a single facility could provide. Maybe if you decided to have an orbital facility 20-40km long to provide ample docking ports it might fly, though as anyone who's been to a busy airport with people movers on it, the more spread out the facility gets, the more time it takes to travel between gates (or docking ports) - which includes both cargo and people.
Balkanized worlds, much like multi-clan Aslan worlds, would each have their own starport - possibly both high and down. Starport/spaceport nomenclature are better defined with the purpose of the port and not if it's the mythical port-of-call for the system. The idea of a single port also fails for any system with any significant settlements or facilities beyond the primary world, any system like Trin that doesn't even have a planetary component, or a system (not just the planet) that is balkanized.
The article you may be referring to is from JTAS #3 - Jump Point to Port. It does discuss delays and such in getting a docking slip and some die modifiers for priority. This concept is, primarily, related to the idea of a single port, and 50k Dton behemoths sharing docking locations with 100 Dton minnows - and the idea that bigger generally always gets priorities. Air traffic is similar, to an extent, is similar since air traffic control will allow a Cessna to land in front of a Airbus 380, but these are the rule and not the exception. Small craft very rarely land at major airports since facilities are not meant for them, and servicing and such takes longer. And you won't see a 100ft yacht docked at the pier of the Port of Los Angeles in place of a 20,000 TEU container ship.
I do see the exceptions, however by default they would be exceptions and not the rule. Since it's always been said the Imperium survives on trade, ensuring that it's run efficiently should be the default assumption. And that's why I used the label as I did.