Ignore it if you'd like. As always, that it your perogative. It says effectively Law Level 1. You asked where it said that. Which "imperium" sourcebook? What is its name?
So, in your opinion, nothing is Traveller rules and everything is setting rules? Is that your stated opinion? Because, the last time I checked, Law Level exists as a Traveller rule to define your universe. Without the UWP, whatever you are using, are not Traveller rules. If you use UWPs, you are using the Traveller rules. You do understand this? Yes? That the Traveller rules are the framework, such as UWPs, skill ranks, task resolution, time frames for personal combat versus ship to ship combat. Yes? Then you use that framework to define your individual universe, such as the specific UWPs of each individual world, the stats of your custom-designed ship, what specific skills and at what rank for each character, what actions can be taken in combat (limited by the settings available tech), etc. You can understand the difference between these two concepts? Yes?
Traveller rules are the same regardless of the setting. Setting rules are only true within their setting. See?
The UWP framework exists in all settings (Traveller Rule), Planet "Mungo" with a specific UWP exists only in your universe (Setting)
Rules to "create your own setting" are obviously base Traveller rules. Why? Because you cannot have setting rules with no setting, and as you stated above, you haven't created a universe (setting) yet. So clearly the rules are to build a setting, making them universal rules and not "setting" rules.
Is English not your first language? That would explain the problem. My wife's English is her second language, and she has problems from time to time understanding English language and grammar.