We have similar background 
Head geometry and listening position/speakers position geometry are only of relevance and significance for crosstalk reduction. REW/Rephase/DIrac do not address this.
There’s nothing “idiophile” at all about it: cross-talk cancellation DSP have been around for years.
The simple idea is that your left ear should receive what comes from the left channel, and nothing from the right channel (and vice versa)
The trick is to compute a DSP that is preserving the signal from the left speaker and “cancelling” the right speaker at the left ear position, and vice versa. This is done by a matrix filter where data from the right channel is delayed, inverted, reshaped in frequency, and injected into the left channel in order to “cancel” what comes from the right channel to the left ear.
Why delayed: in a typical configuration your left ear is farther from the right speaker than the left. This delay depends on your sitting position relative to the speakers and distance between your ears, that’s why they need to be measured.
Inverted: so that the sound waves cancel and do not add (NB: if you edit your HAF filter you can see the impulse response of the x-talk is actually 180° out of phase with the room correction filter)
Reshaped in frequency: this is more tricky. Most likely your left ear is not directly pointing at the right speaker. It will hear whatever contribution the right speaker has on the reverberated field (this is the core of the HAF room correction) AND what will be diffracted by your head, which is frequency dependent (cf. https://search.yahoo.com/search?ei=utf-8&fr=aaplw&p=diffraction+sound+waves) . Typically, with my 23cm high head, frequencies below 1700Hz are diffracted (and heard directly by my ear), and the frequencies above are not diffracted and do not reach my hears directly. If you have a shorter head (let’s say 20cm), this transition frequency is 2000Hz. The X-talk cancellation filter has therefore to match the frequency “response” of your head as it is diffracting the sound waves coming from the right speaker. And this response is quite dependant on your head geometry, both vertically and horizontally. That’s why there is a need to measure it to “tune” the filter. With typical head sizes, diffraction occur in the 1.5k-2.5kHz area, this is incidentally where the ear is the most sensitive.
Regarding the size of the sweet spot where the X-talk works well: I’d say in my case it’s a 30cm radius “bubble” around the sweet spot. At greater distance, this is normal stereo listening with room correction (the one you have with the standard HAF filter).
I hope the technicalities above will convince you there is nothing idiophile about the approach of X_talk pushed by HAF. Regarding confirmation bias: if it would be only me to nod and clap hands, I would doubt of myself. Fortunately, there are lots of positive testimonies out there, including some from audiophiles that are far more experienced and critical than me.
Regarding my relationship with Thierry: I’ve said it on another post, but for the record I am not affiliated with him in any way. I paid for my filters. As a DSP-savvy person and audiophile I am enthusiastic about the technology and I am on this forum only to share my experience and explain DRC in general and HAF in particular. Until there is a new solution out there that is outperforming it !