Let's check what the rule says:
3.3.1 Rally wrote:
HOME REGION: A Faction that Rallies in a Region marked with (emphasis added)
its symbol and “Rally”—Arverni in the Arverni Region, Aedui in
the Aedui Region, or Belgae in any Belgica Region—may place
at least one of its Warbands there even if it has no Ally, Leader, or
"at least one" means exactly that: if the normal Rally rule would not allow you to place any Warbands at all in your Home Region, the special Home Region rule nevertheless allows you to Rally 1 Warband there. If OTOH the normal Rally rule would allow you to place 1 or more Warbands in your Home Region already (because you have one or more Allies, a Citadel and/or - if Arverni - Vercingetorix there), then the Home Region rule has no effect at all.