I think when this discussion was raised in the old Aikido journal forum, I remember Stan Pranin also mentioning that this meeting occurred in 1977 or 78 (He mentioned a more concrete date, I remember).
Okamoto sensei himself explained to me why this meeting occurred. At that time, Horikawa sensei's daughter was living in or around Tokyo with her children and although the children were visiting their grandfather every year Horikawa sensei desperately wanted to see them at their home and know how they were doing. As Horikawa sensei was becoming very frail every year, and as his family and student sensed that he will not live for a longtime, they came to the decision that this (1977,78) was probably the last chance to arrange a trip for him, so the whole family and his senior student accompanied him to assist him.
Tadao Ogawa, the former student of Shioda and who became a student of Horikawa then informed Yoshinkan about the trip and Shioda and Yoshikan arranged for a demonstration of Kodokai. Okamoto sensei said that Yoshinkan paid full respect to Kodo Horkawa so that they treated Kodo Horikawa as a Kyoudai deshi of Shioda's teacher Morihei Ueshiba and out of respect no demonstration was done from the Yoshikan side. So Okamoto sensei had not the chance to witness the ability of Gozo Shioda. He said that he saw a demonstration of Gozo Shioda after he relocated to Tokyo and was literally astound of the high level of "Aiki" and since then wondering from who he got it.
I agree seeing that you cannot get much by just watching old films. And this conversation was done in the 1989, long before internet and youtube. Martial arts video was very rare, and even if they were sold sometimes, they were ridiculously expensive some even sold at the equivalent of USD 200 or even more.
At that time when images of Ueshiba was shown on TV, it was usually that of Ueshiba in his 80`s and tossing around young people even without touching them. I do not think that Okamoto sensei (or other Daito-ryu practitioner AND Aikido practitioner) had many chances to see other images of Ueshiba in those days, aside the Aikikai students maybe.
And I have other hypothesis, that maybe when Okamoto sensei was referring to "Aiki" he probably meant "Aiki" a la Kodokai style. He detailed me about how to discern whether a person really applied "Aiki" to the opponent or whether it was different. If an opponent has been applied "Aiki" in most cases (I remember him saying 70% or something like that) that his knee begins to trembling as he looses instantly power in the knee and that your heel begins to rise so that you are standing on your too.
But I became to think that the above description is probably only a partial side of "Aiki" and that these are unique to Kodokai, but probably that this is not the whole picture.
Anyway I think it is interesting is that the Yoshinkan black belt seminar, in which Shioda displays very Kodokai-esque techniques began in 1978 (this is officially stated in Yoshinkan history).
Don't you think the timing is very interesting? Black belt seminar were very un-Shioda (and un-Yoshinkan) like who always emphasized the basic. I do not think that Shioda got it from Horikawa (or anyone else) other than Ueshiba, but as one of his student (Ogawa) went over to Kodokai stating he found the true thing, maybe the black belt seminar began as a kind of "Oh, but I can do that too" demonstration....