It works great if the chord is far away from the center (and also the angle is less than a half turn). If the chord goes through the center, it doesn't work at all.
It was essentially the same reasoning as for using `-ffast-math`. Yeah it's not "correct", but users can't tell the difference and it has a measurable performance benefit.
To be clear, I'm not questioning using slerp() at all, it definitely has a role to play. I'm just wondering about using it by default as implied by the post.
I think it's useful to think of it as a separate category of "wrong" than -ffast-math, which is "wrong" because two expressions that are mathematically equivalent over real numbers, like a + (b + c) and (a + b) + c, are not equivalent over floating-point numbers.
Using lerp in place of slerp is like using x in place of sin(x). (The small-angle approximation) They're not equivalent over the real numbers.