No, it was a bug with BFG being flagged as Bow/Main Hand. If you placed a Bane Bow in your ranged slot, the code flagged all 'bow' damage as Bane damage. Meleeing with your BFG still counted as 'bow' damage, therefore it got the bane flag.
It's the same code confusion that allowed you to use Trueshot to get double damage while melee'ing with the BFG.
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