You've slipped in a double negative, so let's rephrase for clarity:
From: "The conclusion of the OP study is not that jet stream winds do not have high energy densities--they obviously do."
To: "The study starts from the known high energy density of the jet streams."

Right.

Then you say that the winds achieved those energy densities due to lack of friction, not due to stronger force.

That can't be stated because they did not look at the way extraction would affect the input of wind generating energy into the jet stream system. They essentially took a steady state situation, changed one variable and looked at the downstream effects - without considering the upstream consequences at all. The conclusions, while interesting, have no predictive validity at all, a point Miller etal are very clear about themselves.

I'd recommend reading the reviewers comments in post 24
http://www.democraticunderground.com/discu...