This is what has always confused me about this rule. The rules states "Children should be supervised. Guests under age 14 must be accompanied by a Guest age 14 or older to enter a theme park, water park or NBA Experience. To board an attraction, children under age 7 must be accompanied by a person age 14 or older." If you read the last sentence alone, it sounds like 7 and up can go on a ride alone. BUT the first sentence states that children should be supervised, and if they are entering a ride queue alone, they won't be. One interpretation could be that when they say children age 7-14 can "board" a ride alone, they mean can sit in their own ride vehicle (e.g., a car on a roller coaster, boat on splash mountain, or individual dumbo), but they still have to have an adult with them in the queue for supervision. That would be consistent with their approach to under-7s, which is that someone age 14 or up must be sitting in the same ride vehicle (and sometimes even the same row, depending on the ride).
Of course, this is probably all reading too much into it and the CM was just wrong--wouldn't be the first time a CM didn't know the rules.