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That is an interesting question; did Volta discover that two metals separated by a brine-soaked blotter produced an emf (not yet defined) and verify that it did indeed do so or did he invent the voltaic pile? Probably either description would work.
In any event, a milestone doesn't have to be for an "invention. To quote from GHN
"Milestones recognize the technological innovation and excellence for the benefit of humanity found in unique products, services, seminal papers and patents."
This milestone qualifies in more than one of those definitions.