It's the distinction between it being a have-able thing, versus a thing medically conjured into being. It's also the difference between it being a thing, and it being a thing emulated to get disability or benefits. 

>I know you can't prove a negative, but saying definitively that it "has no existence without diagnosis" seems too strong

Either way, if you can't diagnose a thing, you can't say anything about its existence in terms of disease. All you can say is "I see the following things."

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Ok, that doesn't seem like a particularly important distinction. It's entirely possible they "have it" and we just haven't developed the right approach for clearly identifying it in the body.

I know you can't prove a negative, but saying definitively that it "has no existence without diagnosis" seems too strong. However, "has no *known* existence without diagnosis" would be fine as far as I know.