So I've been through ever mention of the word "focus" on theoryland. Brandon uses the word only twice in a way similar to what we've been using. Both times, once in the annotation for one of the Mistborn books, he calls the metals a focus for the Metallic Arts. I can't provide quotes because I'm at work and copy and paste doesn't work here.
So it seems to me that focus is a real concept in the Cosmere, and rather then trying to move away from the term we should work to better understand it and reach a better consensus for what the term means. Since Brandon called the metal a focus for the magics on Scadrial, and the quote provided in the OP seem to deal with using metals as a "nozzle" to shape Investiture into an effect, I am inclined to think that Moogle's definition of foci is pretty close to the mark.