Could this actually be a response by Knuth? I doubt it.

It doesn't read like him (i.e. I don't think he would call it 'Donald Shell's idea'). Also you don't have to print and mail Knuth. He doesn't use e-mail, but his staff does. I know because I myself have interacted this way with Knuth before.

Overall there is no other 'proof' that this is actually a response by Knuth.