It's not an analogy of the research itself, it's an analogy of how someone might fallaciously use the research to declare that CO2-based warming is not happening.
Sure, it's possible the symptoms are due to some other unclear cause, but it's sufficiently unlikely that the person is being unreasonable.
I think the reductive view is "because the medieval warm period was caused by ocean currents all warming COULD be caused by ocean currents and since ocean currents aren't what the model says (about modern warming) the model must be wrong therefore AGW is wrong because it can't encompass this warming event"
Sure, it's possible the symptoms are due to some other unclear cause, but it's sufficiently unlikely that the person is being unreasonable.