I agree that the distance from your head to the speakers should be equal.
Equal to what?
But I see no reason at all why the distance between speakers should be equal to the distance from one speaker to your head.
Try it out. There's always an optimum distance between the speakers and I prefer to be further away from the speakers.
Take as example, good monitors that have a good omni characteristic. You need to place those differently from most speakers. Besides, you can turn the speakers inwards, or not. And look at what is considered ideal (if you treat the room): speakers mounted flush in the wall. In that case, I've never seen a triangle setup.
What do you mean "monitors that have a good omni characteristic"...?
AFAIK...speakers fire in one direction (not counting the LF ports/radiators you see on some that are in the rear or side).
There may be a difference in dispersion from one model to another...but "omni" is not a characteristic I would describe a speaker with, because that implies firing in all directions pretty evenly.
Also...it's not just about turning them in or not. If you're far back from a set of speakers that are close to each other...you have minimal to no stereo spread.
The two speakers close to each other with you further back become a mono point source to a great degree...regardless of how much you turn them in or not.
AFA the flush mounted speakers...the triangle is between them and your centered mix position.
That's what the whole Left-Right thing is about, with you in the center and back the same distance as the speaker spread. If you are in-between them on the same or close to same plane...you end up with a "hole" in the center of your image.
Like was said earlier...the toe-in has a range, usually 20 to 45 degrees...with the 30 degree being considered by many as the optimal point to start from, and then depending on the dispersion of your speakers, you can toe them in/out within that range and still maintain a credible response from them...but ideally, you want to be sitting just inside the apex of where the speakers are aimed at. If they are toed in and pointing at your face...then you are listening to the sound of the speakers and getting their full hype...if they are pointing way to far in back of you...you're missing out on some of the information.
Using the equilateral triangle eliminates a lot of the potential issues...and the spread of the speakers should be at a specific distance from each other (and also from you) for the best stereo imaging. If they are spread only a couple of feet apart...how do you then get "inside" the apex...and if you are further back, you're just hearing the speakers and not your mix correctly.
Look...you can imagine whatever "myth" you want, and you can buck the equilateral thing...but if you compare the equilateral setup with proper speaker distances to some unorthodox setup with speakers real close to each other and toed in/out to some extreme angles, you will easily hear how much better things sound with the former, and how much better you stereo image is when you sit at the correct spot inside the triangle.
AFA the links from GS...I really don't care, that place has a hard time agreeing on too many things. Look at the links I posted earlier from AES and other sources.
The other thing about the room size...well yeah, if you have a 10' x 10' room...it's pretty hard to do a 67" speaker spread and put mix position at the 38% mark...etc....but that's got nothing to do with how things should be.
Being in a small and/or bad sounding room doesn't somehow make unorthodox speaker setups valid...just because you have no other choice. So that's not an argument anyone should even make...though I see people here do it all the time. They use their limitations (whatever they are) to create a new "valid" perspective about how things should be. That's just called living in denial.
The point here is that it's not a "myth"...and yes, sometimes you have no choice but to deviate from the accepted norms...but the norms should always be the target, and not what you had to deviate to.
This link you posted uses EXACTLY the equilateral triangle setup:
Surround Sound Speaker Placement | 5.1 & 7.1 Setup Guide
Anyway...here it is again, the link I posted on page 1....look at it, try it out and hear for yourself... and then do whatever want. It will only matter to your mixing.
https://www.carltatzdesign.com/acoustic-tools/the-null-positioning-ensemble.pdf