Hmmm, so apparently the clip had someone gotten the field order set back to “lowest first”, which apparently overrides the project settings.
However, changing it back to None (progressive scan) greatly reduces the motion blur, but at the same time it shows up badly interlaced (the original clip was interlaced). It’s weird because it doesn’t show up interlaced in the preview window, and even weirder because After Effects has absolutely no problem rendering the same clip with no interlacing or motion blur.