I’m sure what it did was roll not slide. It is common for automatic transmissions to hold temporarily on an incline but if the car is jarred in the slightest the transmission will not hold. I have had this happen before. I parked and forgot to set the brake. As I close the door the car started to roll. Thankfully it didn’t go too far before hitting a high curb and stopping.
Automatic transmissions with their parking gear give too many people a false sense of security, my uncle would re-build transmissions and showed me that the parking gear is a tiny peice of metal that locks into a saw tooth like gear inside the transmission. When I saw that the parking gear was that tiny I never stopped used the actual parking break in every automatic transmission car I ever drove.
You are in some ways better leaving a manual in gear than using park on an automatic transmission.
The park pawl flashback from 3rd year HS auto mechanics class. One of the few things I remember from the automatic transmission segments.