When he says, "Aikido is misogi", he means the practice of aikido is an act of spiritual purification. Hopefully he is right and we are all better people for it.
The practice of Aikido, if done with the proper spirit
, is misogi. It isn't inherently misogi, I think. There needs to be a degree of pushing through or going beyond for it to be true. There is an element of effort and there is an element of being totally "present". Then you can say that the practice is a form of misogi.
I also think ones attitude comes into it. Practicing and acquiring technique merely to defeat another person is not what was meant. Practicing as a form of self cultivation was. In other words, it would be possible to train with the wrong set of goals and it wouldn't be the kind of misogi O-Sensei meant.