Evidently a garbled allusion to the 30,000 such boys mentioned in Arrian, Diodorus Siculus, Plutarch, and other sources on Alexander the Great.
The present version is wrong in its location as well as the figure: having been organized in Baktria in 327 BCE, the force was put through its paces in Susa (one of the Persian-Empire capitals) three years later. See generally A.B. Bosworth, Conquest and Empire: the reign of Alexander the Great (Cambridge 1988) 272-3.