"90% of Aikido is Atemi" is the correct aphorism.
Now some people equate Atemi to strike which I think is a mistake. This is what confuses people.
I've been taught that Atemi is 'capturing the mind', which can be achieved by a strike or other methods. The point is to disconnect the attacker from their original intent.
Sensitive ukes don't usually need to be physically struck to react to Atemi. Very focused ukes sometimes need a solid whack to get moving. the Atemi "strike" can be the same for both, just with a different ending.
Again, what I've been taught on Atemi is not the same as what I see used in the forum. The point is to take the mind, therefore Atemi does not have the same goal as a Karate strike.
For example if your Atemi is too fast it might as well have stayed in your pocket. It won't do the trick, Uke has to sense it to react to it.
When I look upon Atemi in this manner "taking the mind" I see it in all styles of Aikido I know of, with variations of hardness or softness that are particular to each style. And it's present in all techniques I see.
So 90% doens't sound hairbrained at all.
In my opinion practicing Atemi on a bag is not possible. The Bag has got no mind.