Mark 9:11-13 is an interpolation in Mark based on Matthew:
11 And they asked him, “Why do the teachers of the law say that Elijah must come first?”
12 Jesus replied, “To be sure, Elijah does come first, and restores all things. Why then is it written that the Son of Man must suffer much and be rejected? 13 But I tell you, Elijah has come, and they have done to him everything they wished, just as it is written about him.”
(the equivalent passage is absent in Luke)
Note that there is not the expression:
...and the discipled understand that he was talking about John the Baptist
...since the readers knew
already that collateral effect having known it from the Matthew's original version for that interpolation:
Matthew 17:12-23
12 But I tell you, Elijah has already come, and they did not recognize him, but have done to him everything they wished. In the same way the Son of Man is going to suffer at their hands.” 13 Then the disciples understood that he was talking to them about John the Baptist
Hence: in proto-Mark John the Baptist is
not Elijah.