Could not talk like this must refer to content, not to manner of speech, for example, “could not say things like this” or “could not communicate a message such as he has communicated.”

How could a demon give sight to blind people? is more literally “Can a demon open the eyes of blind men?” In Greek the expected answer is “No.” This exclamatory question may be changed into a statement: “A demon could certainly not give sight to blind people” or “… cause blind people to see.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .