The k that e_power_function wants is in radians^-1 (say).  But the real k that GalSim will be using is in arcsec^-1.  So (scale * k) is in radians^-1, which is passed into e_power_function.  Then, as you say, that results in a value with units radians^2, so we need to multiply by (scale*k)^2 (which has units radians^-2).  So I think the line is correct.