I mean there is a similar effect as with a stereoscopic camera. Both lenses are pretty close to each other, still viewing angle is different, and overlay of branches in the foreground and houses in the background will differ between the right and the left stereoscopic picture.
If you use a non-reflex camera, the viewfinder will show a little different overlay of post and house than the camera lens will record - and an on-top-viewfinder will show an even bigger difference in overlay of foreground and background.
To compensate you had to look with your eye only, put the camera to your head and then move your head accordingly to compensate the camera lens being offset to the non-reflex viewfinder.
The further distant your subject is, the less it will mater but i was surprised how well the fence post is placed into the middle of the garden house in the recent picture - and the fence post is pretty close to the camera.
Offset of an on-top-viewfinder may be problematic here - even if the on-top-viewfinder does have proper parallax-compensation adjusted. Proper parallax compensation will show you what part of your subject will be on the neg, but it cannot show you if the fence post is in the middle of the garden house, because the non-reflex-viewfinder will look at the subject from a different angle than the camera lens.
This picture here is upright format, meaning the viewfinder was to the left or right of the taking lens, depending on how HCB did hold the camera.
If this picture was horizontal format, a non-reflex-viewfinder, on-top-viewfinder with or without proper parallax compensation wouldn`t matter - assuming the non-reflex-viewfinder is right above the taking lens (which it apparently isn`t with an old Leica, but some other cameras do, like a LF camera with on-top-viewfinder for example) - it wouldn`t be a problem to place the fence post.
But with upright format, a fence post closer to the camera and an on-top-viewfinder this should become problematic, whether parallax compensation is properly adjusted or not.