Gimp is a raster editor and works with pixels as a minimum unit.
The problem is you are working with a small part of an image. The depth of the path / selection is only 9 pixels. Using Fill -> Path the fill is complete pixels as (1)
You can bucket fill and get some anti-aliasing which when viewed in the context of a large image "smooths" the diagonal edge. (2) The zoom factor for this is 3200 % looks very different at 100%
If you are hoping for a straight diagonal edge - no steps - no antialiasing, then you are going to be disappointed.
The problem is you are working with a small part of an image. The depth of the path / selection is only 9 pixels. Using Fill -> Path the fill is complete pixels as (1)
You can bucket fill and get some anti-aliasing which when viewed in the context of a large image "smooths" the diagonal edge. (2) The zoom factor for this is 3200 % looks very different at 100%
If you are hoping for a straight diagonal edge - no steps - no antialiasing, then you are going to be disappointed.