According to the description on page 159 of Prof. Lipo's book "Introduction to the AC Machine design":
It is important to mention that the harmonic leakage is important only for wound rotor machines. When the induction machine is equipped with a squirrel cage the harmonics in the air gap produced by the non-fundamental components of MMF will each induce a component of rotor current which will tend to “short” the harmonic inductance. Although belt harmonic fluxes still remain in the gap, they are now very small, being the summation of the harmonic stator fluxes and the opposing harmonic rotor fluxes.
However, I could not understand why such induced higher order harmonics rotor current, which cancel out the effect of higher order stator MMF in the case of squirrel cage rotor, cannot happen for wound rotor. After all, the current in the wound rotor is induced by the stator MMF, just the same as squirrel cage rotor.