It’s not to be taken that our John Brown was Dumbledore’s student. Instead
there was some other John Brown who, moved by the terrible suffering of a
hellhole under the control of some Dark Wizard, went and attacked the Harp of
Faery in an attempt to free them all at once—and died in the attempt.
Eliezer says on r/hpmor that it is a reference to the John Brown of Harpers Ferry. Responding to someone pointing out time-and-place difficulties, he says: