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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."