> the maximum deviation between G1 and G3 is only 0.1mm

In a small 100x100mm box, with a 12mm fillet, G1/G2/G3 corners already have a visible 0.5mm difference. What gives it away is the lack of a hard transition between the flat surface and the corner, that's very noticeable on a reflective surface.

On the mechanical side, I think the effect they refer to also comes down to that transition line - going from a straight line immediately into a curve (G1) which adds lateral forces, vs easing into that curve over a few more steps which avoids jerking the print head.

I may have measured incorrectly in the provided model then. That's still pushing things at 3D printer scales, especially when you don't have a polished surface. I also think an internal corner might be more noticeable by feel.