All of you are correct!
As much as possible we tried to make the rules work the way things should work and be intuitive. Although in this scenario, it may seem a bit peculiar!
As Tom suggested, the sweeping attack could be a vertical strike which hits the second creature while swooping (up or down). That seemed better than attaching a special "realism" clause in which "sweeping can only hit 2 target 2 creatures which either both have, or both do NOT have, the Flying trait."
As much as possible we tried to make the rules work the way things should work and be intuitive. Although in this scenario, it may seem a bit peculiar!
As Tom suggested, the sweeping attack could be a vertical strike which hits the second creature while swooping (up or down). That seemed better than attaching a special "realism" clause in which "sweeping can only hit 2 target 2 creatures which either both have, or both do NOT have, the Flying trait."