Almost correct. You typically need 2-4 transmitters to emulate the system. So you modulate one or two channels under test and modulate the rest of the band with a single modulator and use some decorrelation tricks to be realistic. Then you scan your channels under test through the whole band. This is in typically a lower bound of performance, i.e. a real system would likely perform better. As you said, using individual transmitters is economically unfeasible even for the best equipped industry labs.