There is no doubt that Shioda trained - a little - with Kodo Horikawa. It's not a secret either. I wrote to the Yoshinkan and had a very clear email exchange with one of the younger shihan. He said, essentially, "yes, everyone knows that." Horikawa came to the Yoshinkan and taught at least one open seminar, and then on several occasions (I've heard a few and up to ten or so), Shioda and some senior students met with him behind closed doors. I then wrote and asked if Shioda sensei's technique changed from that point. The young shihan said he would check with some who were there. He got back to me and said that Shioda sensei, per the senior shihan, did not change before and after. (Which could be true and could be not). I do have independent, first hand, confirmation that Shioda, himself, was somewhat discomfited when his association with Horikawa was made (sort of) public.
A couple of points - descriptions of Ueshiba, when he was NOT trying to do his "aikido" sound remarkably like Daito-ryu. Specifically, his demo at Kenkoku University in Manchuria, where Ohba Hideo, (Tomiki's 1st deshi) believed that he would be disrespectful if he didn't try to attack all in, trying to destroy Ueshiba. Ohba was highly ranked in judo, in karate and had extensive training in various weaponry as well.
Tada Hiroshi stated:
Ueshiba was furious with Ohba, because, he felt, he was not able to demonstrate what HE thought was important. Ohba recalled that Ueshiba stomped off the mat, yelling at him, "Omae wa baka."
He was mollified, however, when Sonobe Hideo, the most wonderful naginata sensei in Japan at that time ,approached him.
I highlighted the one section of the last quotation. She'd seen him and thought she knew what he could do - but he, unleashed, showed her something different. In other words, we very well could be making a mistake about Ueshiba simply basing things on his films, where he was showing what he "wanted aiki-do to be."
Which leads to a final point. Tenryu, the ex-sumotori, stated that Shioda was closer to Ueshiba's technique than anyone. Yet when we look at Shioda, we sometimes not this "sharp" and subtle technique which is, those in the know say, more Daito-ryu than Ueshiba's aiikido.
So maybe Shioda, too, was guilty of not doing Ueshiba's aikido ("That's not my aikido!"). But he very well could have been doing what he had actually learned from Ueshiba, something that Horikawa confirmed rather than it being new to him - whether or not Ueshiba himself wanted that to be his legacy.
The argument is not whether or not Shioda studied with Horikawa for a few classes - he did. The Yoshinkan states that openly. And further, I'm not denying the possibility that a few sessions with Horikawa somehow opened up a new world to Shioda. But one of the least attractive aspects of Daito-ryu society is how one teacher will claim another person as a student, after one or two lessons. Horikawa, himself, took six of his students to learn some DR jujutsu (he'd concentrated on the aiki with Takeda) from Sagawa, signed a book, and from that day forward, Sagawa stated, "Horikawa's my student."
In sum, then: No denial whatsoever that Shioda studied from Horikawa. But the evidence I've cited above suggests the strong possibility that Ueshiba could do that style as well - but he eschewed it for his own reasons.