> On the contrary, the actual rules and methodologies for credit score calculations are not documented anywhere public.
The exact formulas might not be, but the general factors aren't exactly a secret. Hell, the "free credit score" service my bank offers even provides a simulator tool to estimate what would happen to my credit score if I did certain things (eg. get a car loan, or pay off an existing loan).
Saying essentially "just don't do bad things to your reputation" is at most a very hand wavy "rule of thumb", but it's not a codified "rule" of the sort we should expect to see of such a consequential system.