Resonant amplification of Rossby waves along a circumglobal jetstream was recently hypothesized as the underlying mechanism for the occurrence of extreme weather in observed episodes with large wave amplitudes. An important part of the argument is based on refractive index theory in the framework of the linear barotropic model. The approach makes a number of assumptions and approximations with the goal to diagnose the existence of a zonal waveguide and, hence, the potential for Rossby wave resonance. The current paper compares this approach with a recently developed direct numerical method that makes no further assumptions given the chosen framework and is, hence, considered as more trustworthy. The comparison indicates that the occurrence of waveguides as diagnosed from the refractive index method is both qualitatively and quantitatively inconsistent with the occurrence of resonance in the direct numerical method. It is concluded that the previously-used waveguide diagnostic is not a reliable basis for detecting Rossby wave resonance. The code for the direct numerical method is publicly available to encourage its use in future applications.