This is something that has been noticed before mathematically, but hard to do in practice. The article is maybe a little confused. The limitation on size comes

This is something that has been noticed before mathematically, but hard to do in practice. The article is maybe a little confused. The limitation on size comes from the limit on frequency/amplitude resolution - for a given scale and resolution, the need to distinguish states grows combinatorially. It can be overcome by making the system smaller. Eventually you can rely on quantum effects to help, when you are no longer emulating a quantum system, you've basically built one.