Hi Dimitriy, No, it's certainly not a mistake! The "did you?" tag means, "is that right?" We use this commonly and naturally. Mr Keene really wanted to know. If we use the usual tag-question pattern ("You wondered about that, didn't you?"), then it means Mr Keene already had an idea about the situation, and only wanted confirmation.

No it is perfectly correct, and is called a question tag.WE use it to get the other person to agree or disagree. For example You are hungry, aren't you?" answer "Yes,I am". " He is nice,isn't he?" Answer "Yes, he is". It asks the person to agree with what you just said.