I'm not a practitioner of ninjutsu, but my experience in martial arts tells me that what you're seeing is actually normal. Here are a few reasons why I believe this:
- The video you link was meant to teach/showcase a technique, not show off the ninja's skill. The important part was the cartwheel, and the sword was only there to show you what the cartwheel was supposed to achieve in a combat situation. Going faster and following through the move would've been more realistic, but less instructive.
- They wanted to avoid injuries. This is a corrolary to the previous point, really. Practicing against a resisting oponent is surely better, but why risk an injury if it can be avoided? They were shooting the video in a really small space, which could've hampered their movements enough that an injury was a likely outcome.
- Most martial arts don't teach new techniques with 100% strength blows. In karate, when studying the bunkai associated with a kata, we would pair up and the uke would strike the tori in a slow and predictable fashion. Only when the tori is comfortable with the countering technique, which can take many practice sessions, will the tori start attacking faster. Many techniques require a lot of coordination and timing for them to be effective, so increasing the speed of execution in small increments ensures that you can work on these elements in a safe and controlled manner.
Because of this, I think the uke here isn't really trying to hit his opponent. He is merely providing an obstacle for the tori to cartwheel over. These videos are meant to be pedagogical in nature, so they take things slow, allowing the viewer to focus on the techniques being shown. They are meant to illustrate combat situations, not be combat situations.
Most likely, these ninjutsu black belts regularly practice similar bunkai at more normal speed, but it really wasn't the point of this video. It's a demonstration, not a fight.