I think that is a bit vague... I mean, how many times did he try to help him correct his ukemi? If that doesn't sink in, occassionally, showing them why taking ukemi that way is wrong is helpful. My instructor commonly showed me why not to do things by lightly throwing an atemi at my face or stomach. Plus, if this reaction was natural for the sensei.... then it can be argued that his mind wasn't taken at all and that he remained in the moment. If it was planned, vicious and done with malice.... then that is a different story. Your interpretation of any of the above statements doesn't neccesarily mean that was how it was, but something to keep in mind.
The uchi-deshi left an opening, Miyamoto took the opening. No pause, no hesitation, no malice. The standard for the deshi (who will be Hombu teachers some day) is much higher than for the rest of us (I made a mistake in my ukemi later that same class and he just corrected me verbally and by pointing to where I lost the connection). The deshi take four to five classes a day, six days a week, and they work out among themselves during the breaks. Miyamoto did the natural thing, the quickest correction to someone who is supposed to paying very close attention to him.