Joan Baez didn't write BIGLY.
Page heard it on an album of hers where she had credited it as Traditional. Page did his arrangement and likewise credited it as Traditional (arr. Page). So if anybody is responsible for the credit problem on BIGLY it is Baez. LZ simply used the only credit they knew for it on the debut which was Traditional. I don't know what else they could have done since Baez had not credited it properly in the first place.
It's interesting how these things get so mixed up even for Zeppelin fans.