Board Thread:Naruto Discussions/@comment-26704954-20150730051953/@comment-26388300-20150730184655

@AsianReaper

You're correct; using the Summoning Jutsu, Madara summoned Kurama, who wasn't sealed in anyone. So Tailed Beasts -- or, perhaps, only Kurama, as no other Tailed Beast has ever been witnessed as Summoned before -- are valid here.