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.
Since the explanation for the claim is essentially pay-walled behind the book, I will just choose to ignore it