Earlier quoted context omitted.
The belief that every child has the same natural ability is as radical. It's radical because goes against all evidence, experience, and common sense - it is ideology taken to a puritanical extreme. There is no more extreme position one can take.
Though I don't think that's the argument. Even those advocating for the end of the program for gifted kids don't believe that all children have the same natural ability. Maybe some of them do, but the main arguments are different and are not actually that "radical". And even if that was the argument, the term was used in the context of "radical egalitarianism". So, the argument doesn't automatically transfer to such…
Given that, there's nothing else to save the word for. This is the limit; the max. So if radical refers to anything at all, it refers to this.