Does Horatio really not know Laertes? He’s seen Ophelia, for sure. But I guess, yeah, Laertes left at the top of the play and I suppose it is possible that he could have missed him.
But it is curious that Hamlet describes him as just a noble youth. It would be more logical to describe him as Ophelia’s brother or Polonius’ son. Horatio should have a sense of those people even if he didn’t have much contact with Laertes.
However – it would sort of give the game away early if Hamlet mentioned Ophelia at this moment. Better to describe Laertes on his own merits and then realize it is his sister going in the ground.
