thanks, that’s very interesting. is it really core part of a copy editors job to know the etymology/meaning in context for such a great number of commonly used words?

Yes. Source: I used to be one. That's how you remember, for example, that it's "free rein" (an equestrian metaphor—letting a horse run without guidance from the rider) and not the currently ubiquitous "free reign."

They are there to edit copy. Fine tuning words is literally the whole job title