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.
It's unfathomably weird of you to frame an article as a source for your claim and then wait for someone to point out that the article does not support your claim before admitting that the source is actually a book that you haven't read.
This whole thread does have me questioning my understanding of cited sources behind paywalls.
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?
You decide whether or not the benefits from you obtaining the information outweigh the cost of obtaining the information. Or you convince Elon Musk to buy the book and leak the EPUB on twitter
> "Well it came from a book. Check it out if you're interested."
> "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