Threstral Question in Harry potter 4 and 5.?

In the Harry Potter books the threstrals can only be seen by people who have seen death. Harry cannot see the Threstrals until after he has seen Cedric murder which is why he can see them in the 5th one. In the last chapter of the 4th book, theres a sentence that reads: "Hermione turned away, smiling at the horseless carriages that were now trundling toward them up the drive..." How can they be horseless still if harry had just seen Cedric die? ......did J.k make a mistake? I think so.