That is certainly a steep claim. As you have already noted, the JBL 2344 produced quite some linear distortion. When you browse the lansing heritage forum, you can see modern day measurements which show resonant notches in the higher frequencies. Here is more material on the old bi-radial, high dispersion waveguides:
http://www.lansingheritage.org/html/jbl/specs/pro-comp/2344.htm
It is realy not true, that they where better designs, but what you are referring to is beamwidth, which is but one factor of a waveguide. The decision for narrowing pattern in the higher frequencies is one solution to create a preferred sloping in-room response, and for many customers of this speaker, it will be perfectly fine, as they will listen to music mostly alone and in a dedicated listening position, which can be covered without a doubt with the remaining pattern width.
However, the technology behind the modern day JBL waveguides is, as you have noted, not a new technology. The waveguides seen in JBL708, HDI-Series or M2 are developments. They can usually be understood as diffraction horns withouth excessive diffraction, due to a progressive rounding, that widens the beamwidth. Into this profile (which differs vertically and horizontically), two more profiles are inset: One in the edges, where a shallower angle assures that the wavefront of higher frequencies does not collapse, one at the throat, where a partial area of the total profile helps to widen the top octaves. This is a sophisticated solution for a baffle-mounted, rectangular waveguide, especially relevant for compression drivers with a bigger diameter exit, not so much for smaller.
However, this is not the only solution, as Genelec with its S360A shows in a elliptical waveguide a similarly extended (16 kHz) performance, although a German reviewer had noted that "some arieness" could be missed, if one was looking for it:
https://www.soundandrecording.de/equipment/genelec-s360a-high-spl-monitor-im-test/