Joe, the applicable rule is quoted below. It specifically mentions that Legends must use Spec tires, but not other Spec classes. In the car classification, also mentions that PT classes will use the tire to assess points, so they are very specific in calling out exceptions to this rule.
3.3. Class legality
3.3.1. Cars (as represented) must meet their respective classâ technical rules, except that all cars may run
any D.O.T. approved tires that fit on the mandated wheel size for their class (unless otherwise specified).
ES and ESR cars may also use non-D.O.T. approved race tires (i.e. slicks) on any wheels. Legends cars
must run the spec tire for their class.