Also, when it comes to the actual costumed cast members, they’re being kept at a distance from guests and generally are not wearing face coverings, so somebody dressed as Cinderella who doesn’t work there will need to wear a face covering, making them easier to pick out.
The other reason the rules were likely relaxed is that, with Walt Disney World being open to limited capacity, fewer guests simply means it will be easier to keep track of the guests that are in the park and deal with any potential issues that could arise from costumes in the park.