Shioda went to the Kodokai.
Not to side track the OP and overall premise, with which I agree but...
This assertion gets tossed around alot and I've yet to hear/see anything concrete other than it is generally accepted that Kodo visited the Yoshinkan a handful of times and Shioda cleared the place out for him. As Ellis points out, no one really knows what was going on there - whether Kodo was teaching Shioda or whether they were just kicking back and sharing a drink.
A couple points that seem to contradict the "Shioda went to the Kodokai" theory are:
1. Shioda's technique was already well established and respected by the time he had established the Yoshinkan. Tenryu encouraged him to start up a school because his technique was "most like Ueshiba's." And, he took top honors at the All Japan Martial Arts Expo (over Tohei). These events were pre-Yoshinkan and so by extension pre-meeting with Kodo (according to the available lore).
2. How many times do you hear of a budo teacher going to the student? I would think that if Shioda was training under Kodo, he would have visited at Horikawa's place or somewhere neutral, not the reverse. Not implying that Kodo studied from Shioda! But rather, that there may not have been much (or any) budo going on when they were hanging out.