India, Malaysia, France, Switzerland and Germany. Go to small towns in most of Europe and you'll find a significant number of vendors not accepting Amex.
I know it is not accepted in many smaller places. I was reading maybe too literally the nonsense assertion, "accepted basically nowhere outside of the US."
Back when you needed to swipe that magnetic stripe the Amex one was different and required a special reader that almost no one in europe had. Except maybe tourist traps.
Not sure how it works now if Amex has discovered chips and contactless. The payment processors probably still need a separate path for Amex and I haven't seen that logo in a lot of places that do display visa and mc. So maybe it's still only available in tourist traps.