Rolled Fondant has ingredients in it that keep the product soft, retain moisture and prevent excessive hardening. Rolled Fondant, and even Rolled Fondant with a bit of hardener [CMC, tylose or gum tragacanth], do not dry absolutely hard, regardless of how they externally appear.
It can be influenced by environment even when "dry", whereas gumpaste [completely different product], once completely dry, is rock-solid. Homemade gumpaste dries harder than commercially available gumpate.
Fondant is really effected by high humidity. Even if you believe it is "hardened" and the humidity goes way up, the fondant will soften up again. Not a pretty sight. (MMF becomes soft like the marshmellow it once was. Don't forget to add 50% gumpaste if you want it to harden well.)
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