The horn flares I made serve to push the tweeter behind the front baffle to improve time alignment, and increase the efficiency of the tweeter by about 5db in the 1500Hz to 10kHz range.
The tradeoff is a dip in the response higher up at 17KHz, but that's an inherent problem with horn loading. That dip won't be audible for the majority of listeners, though, and certainly not me.

As I said in the video the response is technically better with the tweeter flush mounted (as opposed to horn loaded), but then that puts the tweeter farther out of line with the other drivers. What's more of a problem? Flattest tweeter response or time alignment? It's my opinion that both are fairly minor in the grand scheme of things and one can be sacrificed to improve the other without cause for concern.
There is another factor: the solid walnut horn looks cool. It's a nice contrast with the cherry that the speaker is made from.

As for diffraction, that's often much ado about nothing.
If you have a microscope, you see germs, even the ones that are benign.
If you have measurement gear, you see wiggles in the response, even the ones that are benign.
We simply don't hear minor fluctuations in the response, especially higher up in the frequency range.

Is flatter better? Yes, but that's like asking if a day with 5% cloud cover is better than a day with 7% cloud cover. There's a measurable difference, but certainly nothing you'd notice unless you tried to measure it.

I see guys presenting measurement data where they are picking out "issues" in the response that in reality amounts to nothing audible. Turning you HEAD slightly while listening will disrupt the response more. Sitting slightly lower, closer, farther, or over to one side or the other will all impact the frequency response that meets your ears.
And I won't even go there on what the response looks like in an untreated room...

That's why I included the recorded music - to give you a real example of what the tweeter sounds like after each change. You can judge for yourself whether you hear a difference, and whether the difference means better or worse.

Yes, making great sounding speakers involves doing things right, but mainly it's doing all of the big things right. The big things take it 99% of the way and deserve 99% of your attention - driver selection, box design (or baffle design), driver placement / spacing, crossover design. Only after you've maximized what you can with these core components should you shift your focus to that last 1%.

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