High-value contributors follow the rules and social mores of the community they are contributing to. If they intentionally deceive others, they are not high-value.
Arguing that "doesn't secretly, sneakily break project rules" is an essential component of a quality contributor isn't a "no true scotsman" argument, it's a statement about qualifications
You see where this becomes a religious like argument right? Since it's secretly and sneakily there is no way to measure it. So as far as any other participant knows there is no measurable difference, hence your argument depends on said agents to be 'pure' and 'true', hence the exact definition of the no true Scotsman fallacy.
I hope you see how this quickly will advance from a project being about accomplishing some goal, to a project becoming about humans showing they are the ones writing code. Much like we see in religions where people don't give money to the poor to benefit the poor, but show they give money to the poor to benefit themselves. Hence the game playing will continue and the underlying problem will never be addressed.
The point of the rule isn't enforcement, it's setting standards for good-faith contributors.
Your assumption that all rules must be about enforcement is incorrect. Your assumption that only that which can be measured matters is incorrect. I don't know where this belief system comes from, but it strikes me as profoundly toxic.
By this logic, we obviously shouldn't ban drinking and driving - there's no way to test every driver every time, and presumably those most skilled at drunk driving would be undetectable, so it's really just religious moralism.
"Good drivers don't drink and drive even if they think they can get away with it" is just a no-true-scotsman argument, and thus we should actually encourage people to drink and drive so that they get better at it. Nobody should ever have any standards that can't be automatically enforced by a linter, after all.
Instead of progressing to a system resilient to the fact that you can't know how code was written, you've created a rule that, because it's unenforceable and deniable, must retreat to moralization about what someone does in private.
The same argument applies to not including copyrighted code. I can't possibly know if a contribution is a direct copy of some private code somewhere, but it's still reasonable to have a policy against it.
High-value contributors follow the rules and social mores of the community they are contributing to. If they intentionally deceive others, they are not high-value.