I didn't know the internet supposedly made "dumb people dumber." I thought the internet just made everyone dumber.
Unsurprisingly, Gates agrees that the internet can make smart people smarter. By analogy, the printing press also made smart people smarter because it gave them cheap, easy access to far more information. Since they were capable of processing the information, they were effectively smarter than they used to be. It's equally unsurprisingly that he disagrees about the internet making dumb people dumber.