> the dominant workload for a supercomputer was thought to be physics simulations - and they often benefit from high numerical precision.

It still is. Just like how "mainframe" used to be a very general word, and over time gained a very unintuitive definition referring to a very specific type of computer with a specific purpose, "supercomputer" almost invariably means it's a highly bespoke cluster dealing with FP64 workloads. I don't see anyone referring to these AI clusters as supercomputers, for the same reason they aren't referring to the racks as mainframes.

I wonder if there's a term for this kind of semantic narrowing?

I think the term in this case is marketing.

NVIDIA used to use the term supercomputer more often five or six years ago. It's now fully committed to the term AI factory. I think this is supposed to make you think of their expensive kit as a productive asset building your business rather than an expensive tool for your boffins. Your average business executive is instinctively going to question whether they need a supercomputer but don't yet have the same preconceptions about AI factories.

I don't think that Nvidia single handedly caused the move away from the term supercomputer. But I think they had a hand helping push firmly in that direction.