I’ve been a consistent split keyboard user for a quarter century now. My current daily driver is a Redox, which uses a columnar layout. I got into them when I first started having problems with tendinitis. I feel like they help, but I’m not sure what the science says about it.

Anyway, I’ve always hated that diagram because it’s so obviously hyperbolic. I also use standard keyboards on a daily basis, and while there are some posture differences, the bending to make hands perpendicular to the keyboard just does not happen. Comfortably placing your fingers on the home row requires angling your hands a bit because the fingers are all different lengths. Are there some posture differences? Sure. But from what I’ve seen they’re really quite minor.

What I would guess makes more of a difference is tenting. Which is admittedly only possible with a split design. But also, not all split keyboards do tent.

Also, and this one might be specific to my particular problem, moving keys the thumb strikes to a position that it can reach with less stretching has helped a lot. (I suspect that the space bar in particular might have been the source of most of my woes.) And that’s another variable that’s highly correlated with - but still not the same as - the keyboard being split.