She was saved by her Son’s sacrifice, just like everybody else.
If she was free from all sin then she did not need to be saved by the sacrifice of Jesus.
Remember that God holds all time and space in the palm of His hand.
It would be ridiculous to claim that Mary couldn't have been sinless because Christ hadn't died for her sins yet.
We see the same inversion, or prefigurement, in the OT sacrifices, and the Passover marks upon the lintels.
These were (apparently) effective against sin & death - but only because these sacrifices prefigured Christ, the true Priest and the true Sacrifice.