As I understand, the older open steam producing geothermal systems are falling out of favour mostly because the injected, then superheated water returns to the surface often carrying various heavy metals. The water vapor is an additional concern, although it would not seem to amount to much in the grand scale of things, every little bit you can avoid is a good thing.
When you are after accumulating enough heat to be useful from relatively low temperature mediums, you need lots of surface area and the pipe needs to be smaller diameters for doing the snaking and bending without overstressing, so long lengths would be needed.
The depths are suitable for horizontal drilling, as radii tighter than of 1500 to 2500 ft would overstress the pipe, depending on diameters used, There they have to turn fast to the horizontal as they eventually intend to curve back up to the surface to complete the loop. Oil and gas wells of course don't need to do that.
They hinted at some proprietary thermal transfer fluid, which may only be water or brine with food coloring, who knows, but in general water is hard to beat and, as you say, much friendlier than glycol if it leaks. My guess is its water and M&M's green.
