Arcane Archer's +conjure arrows not only are unlocked at the same level as Archmage's tiers, but they also unlock other Arcane Archer abilities much in the same way additional PrE tiers unlock.
They count as Arcane Archer tiers. By all rights and all but one definition (namely, name.)
That being said, I wonder if one of the reasons why they aren't just transferring Stalwart Defender over to Dwarf is because they're trying to make it a bit more unique than Stalwart Defender?
Or perhaps the people coding the classes have been busy trying to get Druid to work.