Most of the time when you're slipping close to the ground you're going to also have flaps deployed, in which case the MCAS is inoperative. Your AOA sensors will

Most of the time when you're slipping close to the ground you're going to also have flaps deployed, in which case the MCAS is inoperative. Your AOA sensors will definitely disagree, but at least the MCAS won't act on them because it doesn't act when flaps are deployed. If you're slipping without flaps I guess it could still be a problem if the disagreement is outside the epsilon spec. I'm not sure if [normal] slip could ever cause that much disagreement.