CET, showed your post to hubby... he said point taken; allowed that a sword of bronze was not necessarily a good explanation for why a sword might be brown, but was more of a possibility than the rust explanation, FWIW. (To be fair, neither of us is too clear on the time frame involved.) He agrees with you that bronze swords would have been seriously inferior to those of steel & iron (even though steel and iron swords were not necessarily the reliably straight, strong swords we imagine them to have been).

Perhaps it is some corruption or mondegreen, after all. Anyone else have any ideas?