I think that is the key right there. It seems like the sensu.instagram.com was simply firewalled at first and the AWS keys were not changed. He was then awarded

I think that is the key right there. It seems like the sensu.instagram.com was simply firewalled at first and the AWS keys were not changed. He was then awarded the bounty for reporting this bug. Afterward he demonstrated that the AWS keys were another vulnerability, and it wasn't until after reporting this, that the AWS keys were rotated. To me, this demonstrates that had Wes not reported the AWS keys, then Facebook would never have rotated them. I would argue that the fact Facebook found need to take action to resolve Wes' third vulnerability submission, could be considered an admission to its legitimacy as a bug. Therefore concluding that the bug is indeed worthy of a bounty.