Exactly my point ... but a texture sampled by DirectX (or a JPEG opened in Photoshop) does not need gamma correction - it needs lightness correction
Can you explain what you mean there?
We use "gamma correction" to mean going from a linear colour space to a curved one, or vice versa.
If the texture is saved in the (curved) sRGB colour space, but we want it to be in a linear RGB space so we can perform math on it, then we need "gamma correction" (meaning, we apply the sRGB->linear curve).
Later, we want to display out linear RGB values on a monitor, which likely uses a curved colour space, so we again need "gamma correction" (meaning, we apply a linear->sRGB curve, or a linear->"gamma2.2" curve, or a linear->"gamma1.8" curve, or a linear->"gamma2.4" curve, etc, depending on the monitor).