This is very common with online gaming (== gambling) sites, and payment systems; anything where there is obviously money being processed.

Do you have any evidence that this has ever been done in the past?

It's not inconceivable, but it seems like a huge leap.

Google seems to provide plenty of examples. Even if it didn't, I don't understand why you see this extortion as a stretch.

It's not uncommon. There was an article a long time ago telling the story of a DDOS attack on a gambling website, and how a guy who was a philosophy major (iirc) figured out how to beat it, and then formed a company providing the same service. Forgot the url/title/etc, but it was good. Plenty more available with a search.