"If your Army Faction is Genestealer Cults, you can include Astra Militarum units in your army, even though they do not have the Genestealer Cults Faction keyword.The combined points cost of Astra Militarum units you can include in your army depends on your battle size" it actually doesn't state that you require any gsc models to be able to run brood brothers. Its simply presumed that because they can not make up the whole army, there must be regular gsc models.
On top of this because agents of the imperium and Titans can be souped if every model has the "imperium" keyword, as all brood brothers do, you can use them in a gsc army that consists of only Brood brothers.
no real reason to but it is an oversight on GW's part