The 'amide bond being planar' refers to restricted rotation about the C(O)–N bond, not the N-aryl bond.
The restricted rotation about the C(O)–N bond does lead to inequivalence in NMR, depending on the compound: a good example of this is N,N-dimethylformamide, where the two methyl groups have different chemical shifts.*
However, in your compound, this restricted rotation is irrelevant. It is rotation about the N–aryl bond which makes the pair of ring protons H-2 and H-6 chemically equivalent (and H-3 and H-5).†
* At room temperature, at least. If you heat it up enough the two peaks coalesce because there is enough energy to overcome the barrier to rotation.
† They are not magnetically equivalent, see e.g. NMR magnetically equivalent protons for a 1,4-disubstituted benzene ring, although that is a separate matter from what you're asking about.