Additionally, Gadi Schor"s post about Shioda noticing the one day an uchi deshi didn't precisely align the slippers in the toilet reveals his almost uncanny level of perception. I believe that people of the calibre of Shioda and horikawa were really operating on a very refined plane.
I don't think that you even have to go that far. Horikawa and Ueshiba had the same teacher and studied (we assume) very similar things. It doesn't seem at all a stretch to me to imagine Shioda having one (or more) "aha" moments after seeing the same material that he studied under Ueshiba presented from a slightly different perspective by Horikawa. I'm sure that happened to everybody in Aikido or Daito-ryu, it's certainly happened to me often enough. Embellishment aside, that leaves the core assumption (that Shioda got something from Horikawa that he hadn't gotten from Ueshiba) reasonably plausible.