Back to the question of the title, when did we find out it wasn't a thing at all? Who questioned those first?
Hey, where is @satoshi? Why isn't he online anymore? Does he hold many bitcoins? Who is Satoshi Nakamoto?
Why does it matter at all? What will you do if you know who Satoshi Nakamoto is? As a bitcoiner, why can't we stop discussing about it? Why can't we just let this mystery unresolved?
The simple answer to your question is - "No one knows"! And I personally would like to keep it that way only! There's nothing we are going to achieve if we know who Satoshi Nakamoto is so please put an end to such discussions!