Indeed, casehardening results in immediate warp when machining.
The description has several points that are incorrect
Case hardening describes lumber or timber that has been dried too rapidly. Wrong...even when dried at the normal accepted rate,mood does caseharden.
Wood initially dries from the shell (surface), shrinking the shell and putting the core under compression. Partly true, but the shell is also under tension and it is this tension exceeds the proportional limit and moves into the plastic range that stress becomes permanent, which is casehardening. So, it develops very early in drying. If the tension exceeds the strength, then a check occurs.
When this shell is at a low moisture content it will 'set' and resist shrinkage. Totally incorrect. Set occurs at high MCs, not low. The wood is too strong at low MCs to develop set.
The core of the wood is still at a higher moisture content. This core will then begin to dry and shrink, however any shrinkage is resisted by the already 'set' shell. This leads to reversed stresses; compression stresses on the shell and tension stresses in the core. This results in unrelieved stress called case hardening. Correction...set causes casehardening, not unrelieved stress.
A better description is in DRYING HARDWOOD LUMBER.