The 'fore' is not the 'before and after' kind, but the 'foreground and background' kind.
It means shortening the distance between foreground and background.
So the opposite is forelengthened.
Ha yes, true, backlengthened is the true opposite.
I think that Miriam Webster entry in your other tweet is mistaken. It relates the 'fore' in 'foreshortened' to time, as in 'beforehand', but that makes no sense.