I think both can be true. Specifically, Bitcoin mining would itself encourage greater energy production; it doesn't merely skim off the excess energy, as you suggest. It generates its own market forces, it's not merely floating above them.
While that's plausible, I'd still agree with the parent comment in general -- it's true that it's not cost-effective to mine using fossil fuels (you would lose money doing it), and this alone requires a different way of talking about the issue.
