Co-author is so strange. Like, would anyone credit an IDE or even a script generating code as co-author? Even legally, tools don't have the ontological requirement to be credited as author.
There are many reasons: https://news.ycombinator.com/item?id=47164481
Pretty sure it's just because if the tool generates the commit, it auto-adds itself as co-author. Like the iPhone emails that say "sent from iPhone"