That seems to be part of the else path on a swipe. It used to be if north/south/west/east (leaving diagonal) but it looks like north & south have since migrated elsewhere, leaving them to be treated as if they were diagonal.
https://github.com/koreader/koreader/pull/2659
https://github.com/koreader/koreader....lua#L485-L490
Edit: removed in
https://github.com/koreader/koreader...a1a13a70fbce6c
Same true here
https://github.com/koreader/koreader....lua#L404-L428