The graph may be for the largest original Heil. The two smaller ones they now make are a later generation, I do believe. My friend was using the Beyma AMT (with back removed) as a super tweeter on top of his 24 inch waveguided open back beryllium compression driver (110db sensitive, crossed over at 350hz)......but then tried the middle sized Heil driver instead and said it is much better as a super tweeter than the Beyma. Obviously, if you are using it below 10K then any measurement other than flat could effect its sound. By the way, this person also owns the Neo 3 and the amorphous core big Raal and prefers the Heil. His crossover on the Heil is just a cap at 10K so frequencies below that are being reproduced to some degree.
http://esslabsusa.com/heil-amt/ He has the II version (middle size). They reduced the price a lot.
Interesting review and tour of factory: https://positive-feedback.com/reviews/hardware-reviews/ess-laboratories-12-amt-series-loudspeakers/
Rule of thumb is that when crossing two drivers to one another the crossover point is limited by the distance of the acoustic centers. In other words the crossover point must be lower than the wavelength that is the distance of the acoustic centers, or the sound will appear to come from different sources.
For instance a 6" acoustic center spacing would mean that the crossover point would need to be lower than 2,250Hz.
For a crossover point of 10kHz the acoustic centers would need to be about an inch and a quarter apart. That is not realistic. You might as well mount them on the ceiling at that point, as the sound will appear to come from a different source.
Higher crossover points require steeper slopes as well. Using a first order slope at that frequency range would guarantee out of phase cancellation up and down the frequency range that deviates considerably with slight changes in vertical height.
Even trying to evaluate the sound of different drivers in that application is fairly futile. So it is hard to put much stock in a guy saying this driver sounds better than that driver in an application where they aren't being used in any realistic way.