Re: 100 percent efficiency?
I expect they mean it superconducts only at isolated edges. Pack a load of edges close together and they simply aren't edges any more. The electrons in one edge start interacting with those in other edges and I'd guess the whole thing becomes an ordinary resistive bulk conductor on the macro-scale.
There's a similar problem in the Semiconductor industry. SiO2 is a good insulator, but only in bulk. As you start trying to make thinner and thinner FET gates, you eventually get to the point where most of your SIO2 is surface and the rest is influenced by being only one bond away from a surface. At which point it ceases to be a good insulator and Moore's law runs out of road.
Another similar conundrum is the tensile strength of a carbon "buckytube" molecule. Strong enough to build a space elevator ... except how do you assemble buckytubes into a bulk material? What "glue" can you use, that doesn't change the (admittedly large) molecule into something else?