Congrats on the excellent review in Stereo.
The M2 is a serious competitor to monitor loudspeakers from D&D, Genelec, Grimm, Kii, Neumann etc.
As a bonus, the M2 is capable of higher SPLs than the competition and attractively priced.
Similar to the Master Reference Monitor that bears the same name, the wide dispersion in the horizontal plane is the result of the 'knuckles at the throat of the waveguide. These were first introduced in the PT waveguides as an elegant, less intrusive evolution of the traditional pinched throat + diffraction slot.
Diffraction Horn (throat):
Progressive Transition Waveguides:
Image Control Waveguides:
Even though the throat looks quite rough (the leftover particles from the mold should definitely be removed), the JBL 2381, shown in the first image, isn't a bad horn. This is illustrated by the comparison between several horns in the attached paper. While directivity lacks behind in the vertical plane, the horizontal is fine. Moreover, the 2381 loads the driver better.
With regard to directivity and more specifically wide dispersion, an additional comment.
The spaciousness of first side reflections resulting from (very) wide dispersion might be appealing to many, one should be aware of the caveats.
If the speakers are placed close to highly reflective side walls, the off-axis energy can easily cause listening fatigue/annoyance.
It's therefore highly recommended to consider placement and room treatment.