Earlier quoted context omitted.
In this context, they are speaking of electrical efficiency, i.e. the amount of power lost to system impedance during transmission, not some abstract concept like effectivity. The efficiency of a transmission line is expressed as a ratio of power received at one end of the line over the power sent at the other.[1] [1] https://en.m.wikipedia.org/wiki/Electrical_efficiency
The cost still matters because if the losses cost less to replace than the superconducting material costs to install, no one will use it. So parents point still stands. It doesn't matter how high the electrical efficiency is, what matters is cost efficiency.
As with all toy models being applied to the real world, there are important factors to model in that aren't immediately obvious.