So I joust found this brilliant scientific paper called "Replication of Quantum Factorisation Records with an 8-bit Home Computer, an Abacus, and a Dog" which mocks quantum computer researchers for optimising factorisation of prime numbers in a way, so that it is as easy as possible to calculate them with a quantum computer. While you may not understand the technical or mathematical explanations they include a lot of jokes and its very funny to read. One of the jokes is,that they call quantum computers a "physics experiment", which is the explanation behind this joke.
So a supercomputer is a physics experiment? A lithography machine for producing microchips is a physics experiment?
I guess a fair threshold would be "when it can be run and maintained by engineers rather than scientists". In which case, many quantum computers are indeed physics experiments.