That's fascinating! I watched the clip first, then noticed Peter's comment about the venue and the uke, and watched it again. Yes, Yamaguchi Sensei came to the UK in the mid-70s to teach a Summer School (although I didn't start practising for another couple of years so wasn't there on that occasion). The first uke definitely seems to be Kanetsuka Sensei, but I don't get the impression he was being obstructive - to me it seems to be more a case of "if your partner does this, then...". Kanetsuka Sensei was physically very strong in those days. I have heard him say in private that he was able to immobilise Yamaguchi Sensei on one occasion, to the surprise of both parties, but I don't think that is happening on this clip.
Kanetsuka was greatly influenced by Yamaguchi in the years after the latter's first UK visit - I believe that Chiba Sensei, as well as Chiba's father-in-law, Sekiya Sensei, both strongly suggested he studied with Yamaguchi, and this started a kind of revolution in Kanetsuka's aikido, as he became much softer and more sensitive. This process was deepened when Kanetsuka Sensei was seriously ill in the mid-1980s and no longer had physical strength to fall back on.
Kanetsuka Sensei, as Peter notes, puts great stress on aikido having to work against a strong grip - I think this is partly a remnant of his initial training with Gozo Shioda, and partly the influence of Saito Sensei in the 1970s. These days one uke is often not enough for him, and he likes to get three or four big guys
to try to stop him moving. All the same, when I take ukemi for him I am very aware of my own openings when they appear, and I can feel that he is too - he just doesn't take advantage of them when it isn't relevant to the point he is trying to get across. Yamaguchi Sensei certainly didn't refrain from atemi, but it was usually quite a gentle reminder that you were in the wrong place.
I think that trying to infer what is actually happening in a teaching situation can often be very difficult, as while there are in general very different levels of response for the uke, it's not obvious which are available at a given instant (and many are not appropriate anyway in any given situation).