I believe a later teardown showed they are not actually halbach arrays. My thought is that the magnets direct the flux from an inner set of stator poles and the outer set of poles act as a typical reluctance motor. That allows twice as many poles, reducing the torque ripple.
To see what I mean, compare the number of stator slots (54) to the number of rotor sections (6). Each pair of rotor sections form a pole pair, giving 3. This is a three phase motor, so requires 6 slots for a full set of stator pole pairs. That gives us 54/3/6=3 sets of stator poles per rotor pole pair. Thus, there is a set of poles formed between the stator poles which align with the rotor shape. Here, the uppercase are the stator poles that align to the rotor reluctance sections, and the lower case are the stator poles that align to the permanent magnet section. The periods correspond to the undriven phase.
NN.ss.nn.SS.nn.ss. repeat 3 times
Net result: large rotor flux sections and small stator phase angle, giving high power and low ripple. Without the magnets, the adjacent stator poles would be 'shorted' through the rotor.
