I was thinking Einstein, but it seems to be credited to Pete Seeger “Any darn fool can make something complex; it takes a genius to make something simple.”
It's really just about speaking to people in language they understand, and lowering any barrier that might cause them to give up. Language is a powerful thing. If something can be worded in a way that makes people feel comfortable then they will take the next step. Interface design is the same as Steve Jobs understood.
Personally I've nothing against a bit of handholding. Not everyone wants to be a systems developer. I remember when I was trying to get to grips with Linux ages ago. I'd ask a question about how to get something working and no-one would give me a direct answer. Instead they'd say 'go and learn about QT' or 'read up on enabling modules' or some such - which was no help because I didn't understand that either. I always suspected it was a power thing, like priests in medieval times insisting on preaching in Latin which the peasants didn't understand. It used to drive me mad.
Conversely, if I had a problem in Excel, I'd go to the Microsoft help pages and someone would just show me how to do it - and I learned something. Nothing to do with 'dumbing down', everything to do with effective communications.