Imagine being caught in a burning structure. The Aes Sedai weaves a bubble of Air around herself to protect her. She will not burn or suffocate for now, but she has not extinguished the flames, and she will eventually run out of air. This is what the Age of Legends Aes Sedai did. What Nynaeve did was put out the fire.

Ishamael's words imply the impossibility of actually healing the madness, but the possibility to give Lews Therin a few moments of clear thought despite this. What Nynaeve did was heal the madness. Therefore, I disagree with Sidious' assertion that what Nynaeve did was done before, as demonstrated in Dragonmount.