Edit: I saw someone else building a cabin with similar windows recently. They are double pane, they are just some sort of frameless unit, held together by the strip of black material around the edge. Whether they work probably comes down to how well the installer seals around the edge of the window.

And while the sliding glass doors look like they are single paned, look at the quadruple reflection. Isn't that usually caused by multiple sheets of glass in close proximity?

The external framing is also 2x6, so someone was thinking about insulation. I think it's just that after declaring victory on a well insulated house they then decided to heat the outdoors anyway by directly heating things that are out of doors.