This is a lesson in how electricity isn't really a commodity e.g. it's very very difficult to send some electrons from one side of the world to another.
Commodities are traded on type, quantity, and place. Oil of a specific grade at a specific port. Pork bellies (no longer traded) of a specific grade in Chicago. Etc.
If you want the commodities elsewhere, you have to provide for transportation. Same for electricity. Grids (or grid sections) where supply outpaces local demand and transmission to remote grids can hit negative spot prices even when neighboring grids haven't.
That it is treated as such speaks volumes to the craft of the people designing and maintaining the grid.
But all commodities are like this. It is actually pretty easy to send some electrons great distances, or heck at least it's a well understood, solved problem. It's just that those interconnections haven't been built yet.
Heck, oil is probably the "default" example of what a commodity is, but we're now all acutely aware of what happens when moving that oil from one place to another becomes exceedingly difficult.