The bear shots, although cute, are not really all that good of a test, in my opinion. I can see why he couldn't tell which was with extender, and which without. The depth of field is shallow in both cases, really only the eyes on a black-furred animal...there's just not enough subject matter to convey "sharpness".
The eagle shots are more telling. The internal plus external combo of a 1.4x iii extender, looks quite decently sharp to me. Certainly more than sharp enough if you get a good filling of subject size within the field of view, as he did here. It's possible this combo at 784mm, is as sharp or sharper, than the Nikon 200-400, with no extenders at 400mm...lol.
I'm not really encouraged to read further into his review, but if anyone spots anything of interest after reading it all, I'll look.