I think their point is, and I agree with their point, that if we have experience listening at close range to the sound of an on amplified piano being played we can develop a mental database reference of what a real piano sounds like. We cannot do that with multi track pop or with a solo vocalist, unless we can listen at length to a solo vocalist singing or talking in person unamplified.
It's a lot easier to find a piano in a friend's living room or hear unamplified piano at a concert hall, than it is too corral Stevie Nicks to sing to you unamplified in your living room.
That is partly it. It is also if the audition music you use and you draw conclusions from, can you use those conclusions for a broader set of LPs, or is it only more LP specific? That is what I meant by extendibility. If I record Ron Resnick in a local studio on their custom vinyl and play it back, I can use Ron in person's voice as reference. Does that tell me how rock and opera vocals will play on the regular labels? To do the latter, I need to use a representation of opera vocals or Decca, EMI, Columbia, speakers corner, classic records, to get a reasonable sample. So I really cannot say I have auditioned a system for vocals by playing a specific LP unless it is a broader representation. This is beside the point of difference of a girl with guitar vs with a larger band or orchestra, which tests a system more. But what you use for that purpose should also be extendible. Plus, it should test for good vs bad recordings - the latter should sound awful.
I am not saying Stevie Nicks should not be used for audition, but it will only be one LP in a larger sample. As I said, when we played Fields of Gold at audioquattr, we both preferred it on the Opus on Graham. Yet, both of us prefer his Vyger RS, Dava, and vdh more overall and that's how he listens on a day to day basis too, those 3 taking priority over Opus. Therefore if I wanted to listen only to Fields of Gold on that system I would use Opus, but it would by no means be a conclusion I could more broadly extend to other LPs in the system.