The students at our dojo are frustrated at the distinction. Drives them nuts when I say something is simple, which they interpret as "easy". Perhaps I will use the whole quote and see if that helps.
I have often used the analogy of learning to ride a bike. It was "impossible" right up until you got it. You'd pedal along with your parents holding the seat and then crash and burn when they let go... than all at once, they'd let you and you'd "catch it", your brain would suddenly figure it out and within minutes you'd be doing loopers in the street. You could not get on a bike again for years and you'd still know how. But there was no half way up until that point, it was all or nothing.
The aiki work is a lot like that. Once you get the "feel" you can do things that had literally been impossible for you shortly before. It is simple, and on some level effortless, yet right up until the instant when you got it, it was really complex and impossible to do. This stuff is amazing...