In the same way a lot of the other rules on that list are. How are they going to know you are familiar with the code you are committing? How are they going to

In the same way a lot of the other rules on that list are. How are they going to know you are familiar with the code you are committing? How are they going to know it is tested? Sometimes rules are established to set up a common community base line. You are trusted to follow them in good faith. If over time, through code review, questioning or testing, it shows you aren't following the rules then you can't claim you didn't know it wasn't okay.