Yes kids do need to be potty trained before they attend pre k otherwise the kid will not be able to attend or be accepted into preschool.
The only exception is that if the child has bladder issues or is disabled in a way that forces the child to have to wear diapers.
Then the school will make special arrangements for the preschooler to wear diapers but only if it is really needed.
Otherwise you have to have your child potty trained and out of diapers before you send the child to preschool and if the child is still in diapers and not potty trained then you can't send them to preschool and will have to keep them home.
However you really have to get the child potty trained and out of diapers before they start kindergarten because they will not be able to attend kindergarten either if they are still wearing diapers unless they are disabled or have bladder issues requiring them to wear diapers.