How does Frank Miller know what happened to Jason Todd(2nd Robin) in The Dark Knight Returns?

So I'm reading Frank Miller's landmark Batman collection, The Dark Knight Returns. In the series Batman gives the job of Robin to a new girl. Alfred warns him against taking a new Robin by saying something along the lines of "Don't you remember what happened to Jason?" Jason Todd was the 2nd Robin who ended up being killed by the Joker. The thing is, The Dark Knight Returns was published in 1986 and the Joker killed Jason Todd in 1989. So what the heck is Alfred talking about?