The article is mostly an interview with a journalist who wrote a book about Napster. The claim comes from his book. That's the source. I have not read the book. If you have concerns about the methodology you should read it yourself.
"Your claim is poorly supported."
"Well it came from a book. Check it out if you're interested."
"What? It's not free? Hell no I won't pay!"
A contrived example I have in my head "My friend has more definitive proof in HIS book that <claim>, it only costs $1 trillion. Go read it yourself." For this particular thread its a bit disingenuous, but in a general case how does one go about understanding / disproving arguments made like this?
> "What? It's not free? Hell no I won't pay!"
I am also sceptical that my burning questions will be answered in the book, thus I choose to not buy it. Also, the Goodreads reviews don't look too good