These explanations are not entirely correct.
How to find out owner of a city: The most well known way is to look on the disembark screen, will show you the owner of the city.
There are several classes of "owners" for assets, for example "characters" and "factions" can own assets.
The "market" can own an asset, it is held while being auctioned for sale after a death or dissolution.
An asset can be owned by the "Hostile owner" which is the umbrella that holds most hostile bandits.
There is also a rare class of owner called the "NPC owner", but this is very rare any most assets are not owned by the NPC owner. The NPC owner is basically the opposite of the hostile owner, its an umbrella that holds non hostile stuff.
Most assets that people would think are "NPC owned" are infact owned by an admin faction (either Quests or Darkness).
An example of when the "NPC owner" is used is when you unclaim an item, it becomes owned by the "NPC owner". (And items owned by the NPC owner can be claimed by anyone else).
So technically yes, you can build in "NPC owner" cities without permission, but finding an NPC owner city will be very rare. I suspect that if these cities exist, it is a mistake and they should instead be owned by Quests/Darkness. I suspect some very early homeworld cities are NPC owner, and that you could build on free squares in them.
If the unclaim functionality was extended to cities, this rule would be more appropriate.
Will you get CPs for finding a weird rule oddity? no. If you are looking for CPs, you should detail an update to the page to make it less confusing in this forum:
https://www.swcombine.com/forum/forum.php?forumID=167