It's not "any" economic good, it's any "fungible" economic good. The first line of your linked wikipedia article:
In economics, a commodity is an economic good, usually a resource, that specifically has full or substantial _fungibility_: that is, the market treats instances of the good as equivalent or nearly so with _no regard to who produced them_. (Emphasis mine)
That's the definition everyone in economics uses and NVIDIA has never fit that definition. NVIDIA GPUs are not fungible. Intel and ATI GPUs are not replacements as far as AI is concerned. There are zero substitutions. They are not commodities, they are differentiated goods that no one else can produce.
> Having a monopoly doesn't mean the product itself isn't a commodity. De Beers has a monopoly on diamonds, but I'd expect diamonds to fit into most people's definition of a commodity.
Yes because the definition of a commodity is fungibility not the number of vendors. You can go online to Alibaba now and buy a bag of synthetic diamonds that are perfect replacement for mined diamonds. De Beers never really had a monopoly, they just controlled the market for consumer diamonds until the 80s.
NVIDIA has a monopoly because its product is not a commodity.