Tamayo said both of them are NOT cursed, i'm pretty sure of that. On top of that, i don't think she ever specified how her "demon conversion" works, so we don't really know how different from Muzan's it is.
Btw, although it is rare demons can free themselves from the curse, i'm unsure how Tamayo did it but Nezuko probably managed it by not consuming anything since she became a demon, which is likely how Muzan can strengthen the curse, since demons can also become stronger by consuming other humans.