My guess would be the EF-800mm F/5.6FL does affect the isolation of the subject more than open [wide] aperture.
Not quite. Contrary to popular opinion, Depth of Field is not affected by focal length only the physical size of the aperture (when maintaining subject height). To maintain subject height in the image the subject distance must be increased as you increase focal length (in other words, you have to step back when using a longer lens). Now, greater subject distance increases depth of field but a longer lens with the same f-number (eg. f/2.8 ) has a physically larger aperture which decreases depth of field. It turns out the the increase of DoF due to increases subject distance is almost exactly offset by the decrease in DoF due to the physically larger aperture with the result that focal length does not affect DoF.