> What do you think is plausible - that they repeated something that is damn near omnipresent in the discourse, or that they never saw any of the many, many times it has come up and independently came up with the exact same formulation?

Both are fully possible.

We're talking about a simple sentence that sounds a lot like something a primary school teacher or parent of young children would say. "If you won't do/don't/haven't done X, why should I Y?" is very normal moral/ethical phrasing.

I suppose I heard someone say it before I spelled it out in that way, but I am pretty confident I would ultimately have used a sufficiently similar expression, because I have been on the receiving end of the problem.

And that is putting aside the communicative compression of aphorisms. People don't use shared aphorisms to avoid ever doing the thinking themselves; they use them to shorthand more complex arguments for a listener who has already heard them. They are shared macros for agreed concepts. (Using them correctly requires having done the thinking they package up at least once).