Today, if I wanted to test Ueshiba's bullet speed I would put him in front of a gun locked in a vice in order to remove human error or the human urge not to kill people. At some point, I would pull the trigger (or have it set up on a random timer somehow) and see what happens. Assuming the bullet hit the target and Ueshiba was not there we would know the answer. I might even go a step further and put a pressure sensor under his feet to trigger the shot. Of course it would also be videotaped. In other words, I'd remove the human element which according to Ueshiba's words should not have mattered anyway.
There are a lot of examples of people being shot at and not hit. Some of the numbers I've read about in regards to the Civil War are remarkably surprising in terms to how few shots fired actually hit someone. Most of that is probably due to humans unwillingness to kill humans. On the other hand, if you've ever hunted and tried to shoot something you'll understand just how hard it is to hit a moving target. If the story reads correctly then Ueshiba was a moving target.
See the book On Killing
for more information.