Their half-size model would need to be flying twice as fast for the Reynolds number to remain constant with varying length or diameter. This changes the Mach n

Their half-size model would need to be flying twice as fast for the Reynolds number to remain constant with varying length or diameter. This changes the Mach number. So no, in general, you can’t build and test a smaller model and transfer the results to a larger model using similarity, because your similarity model is incomplete. You can either have similarity for the Reynolds number or the Mach number, but not both.