Yes that is it.
Spark plug thread size has always been metric.
I suppose some engine somewhere has used SAE but for the most part spark plugs have always been metric thread.
I don't think so, unless it was the Model-T plug, they're huge.
Another place where established metric measurement is far superior is in threaded fittings.
There is no "bolt thread" "Pipe thread" "tubing thread" "gas fitting"...everything in metric is the same, depending only on the diameter and thread pitch.
If you had the proper 14mm wheel lug stud, you could screw it into a spark plug hole. (not that you'd ever want to) or you can screw a regular metric nut onto a brake line fitting.