The Heroes simply have multiple conditions necessary to secure their assistance: 1) You need to have the Horn (this seems obvious, but you do not need to physically hold or blow on it - it was Mat who is the Hornblower, but it was Rand whom they sought), 2) you need to have the Banner (which declared that you were the Dragon) and 3) you need to possess the soul of the Dragon Reborn (presumably they and the Horn can tell if this is the case). That is probably why it was considered a mythical curio during the Age of Legends - there was no reason for Lews Therin to take the legend seriously, since their society wasn't in such desperate straits as the Third Age world.